diff --git a/tools/nut-scanner/nut-scanner.c b/tools/nut-scanner/nut-scanner.c
index 2288f5cf1e..05e743c6e5 100644
--- a/tools/nut-scanner/nut-scanner.c
+++ b/tools/nut-scanner/nut-scanner.c
@@ -1312,7 +1312,9 @@ int main(int argc, char *argv[])
 	}
 
 	upsdebugx(1, "Parallel scan support: max_threads=%" PRIuSIZE, max_threads);
-	sem_init(current_sem, 0, (unsigned int)max_threads);
+	if (sem_init(current_sem, 0, (unsigned int)max_threads)) {
+		upsdebug_with_errno(4, "%s: sem_init() failed", __func__);
+	}
 # endif
 #else
 	upsdebugx(1, "Parallel scan support: !HAVE_PTHREAD");
diff --git a/tools/nut-scanner/nutscan-init.c b/tools/nut-scanner/nutscan-init.c
index 6b891a61de..170249de2e 100644
--- a/tools/nut-scanner/nutscan-init.c
+++ b/tools/nut-scanner/nutscan-init.c
@@ -159,7 +159,12 @@ void nutscan_init(void)
 			__func__);
 		max_threads = UINT_MAX - 1;
 	}
-	sem_init(&semaphore, 0, (unsigned int)max_threads);
+
+	upsdebugx(1, "%s: Parallel scan support: max_threads=%" PRIuSIZE,
+		__func__, max_threads);
+	if (sem_init(semaphore, 0, (unsigned int)max_threads)) {
+		upsdebug_with_errno(4, "%s: sem_init() failed", __func__);
+	}
 # endif
 
 # ifdef HAVE_PTHREAD_TRYJOIN
diff --git a/tools/nut-scanner/scan_nut.c b/tools/nut-scanner/scan_nut.c
index eb6ca64838..20d1d55e3a 100644
--- a/tools/nut-scanner/scan_nut.c
+++ b/tools/nut-scanner/scan_nut.c
@@ -296,7 +296,11 @@ nutscan_device_t * nutscan_scan_nut(const char* startIP, const char* stopIP, con
 				__func__);
 			max_threads_scantype = UINT_MAX - 1;
 		}
-		sem_init(semaphore_scantype, 0, (unsigned int)max_threads_scantype);
+
+		upsdebugx(4, "%s: sem_init() for %" PRIuSIZE " threads", __func__, max_threads_scantype);
+		if (sem_init(semaphore_scantype, 0, (unsigned int)max_threads_scantype)) {
+			upsdebug_with_errno(4, "%s: sem_init() failed", __func__);
+		}
 	}
 # endif /* HAVE_SEMAPHORE */
 
@@ -356,8 +360,21 @@ nutscan_device_t * nutscan_scan_nut(const char* startIP, const char* stopIP, con
 			sem_wait(semaphore);
 			pass = TRUE;
 		} else {
-			pass = ((max_threads_scantype == 0 || sem_trywait(semaphore_scantype) == 0) &&
-			        sem_trywait(semaphore) == 0);
+			/* If successful (the lock was acquired),
+			 * sem_wait() and sem_trywait() will return 0.
+			 * Otherwise, -1 is returned and errno is set,
+			 * and the state of the semaphore is unchanged.
+			 */
+			int	stwST = sem_trywait(semaphore_scantype), stwS = sem_trywait(semaphore);
+			pass = ((max_threads_scantype == 0 || stwST == 0) && stwS == 0);
+			upsdebugx(4, "%s: max_threads_scantype=%" PRIuSIZE
+				" curr_threads=%" PRIuSIZE
+				" thread_count=%" PRIuSIZE
+				" stwST=%d stwS=%d pass=%d",
+				__func__, max_threads_scantype,
+				curr_threads, thread_count,
+				stwST, stwS, pass
+			);
 		}
 # else
 #  ifdef HAVE_PTHREAD_TRYJOIN
@@ -494,9 +511,10 @@ nutscan_device_t * nutscan_scan_nut(const char* startIP, const char* stopIP, con
 # ifdef HAVE_SEMAPHORE
 			/* Wait for all current scans to complete */
 			if (thread_array != NULL) {
-				upsdebugx (2, "%s: Running too many scanning threads, "
+				upsdebugx (2, "%s: Running too many scanning threads (%"
+					PRIuSIZE "), "
 					"waiting until older ones would finish",
-					__func__);
+					__func__, thread_count);
 				for (i = 0; i < thread_count ; i++) {
 					int ret;
 					if (!thread_array[i].active) {
diff --git a/tools/nut-scanner/scan_snmp.c b/tools/nut-scanner/scan_snmp.c
index 46a5663470..7c415b3e45 100644
--- a/tools/nut-scanner/scan_snmp.c
+++ b/tools/nut-scanner/scan_snmp.c
@@ -1077,7 +1077,11 @@ nutscan_device_t * nutscan_scan_snmp(const char * start_ip, const char * stop_ip
 				__func__);
 			max_threads_scantype = UINT_MAX - 1;
 		}
-		sem_init(semaphore_scantype, 0, (unsigned int)max_threads_scantype);
+
+		upsdebugx(4, "%s: sem_init() for %" PRIuSIZE " threads", __func__, max_threads_scantype);
+		if (sem_init(semaphore_scantype, 0, (unsigned int)max_threads_scantype)) {
+			upsdebug_with_errno(4, "%s: sem_init() failed", __func__);
+		}
 	}
 # endif /* HAVE_SEMAPHORE */
 
@@ -1131,8 +1135,21 @@ nutscan_device_t * nutscan_scan_snmp(const char * start_ip, const char * stop_ip
 			sem_wait(semaphore);
 			pass = TRUE;
 		} else {
-			pass = ((max_threads_scantype == 0 || sem_trywait(semaphore_scantype) == 0) &&
-			        sem_trywait(semaphore) == 0);
+			/* If successful (the lock was acquired),
+			 * sem_wait() and sem_trywait() will return 0.
+			 * Otherwise, -1 is returned and errno is set,
+			 * and the state of the semaphore is unchanged.
+			 */
+			int	stwST = sem_trywait(semaphore_scantype), stwS = sem_trywait(semaphore);
+			pass = ((max_threads_scantype == 0 || stwST == 0) && stwS == 0);
+			upsdebugx(4, "%s: max_threads_scantype=%" PRIuSIZE
+				" curr_threads=%" PRIuSIZE
+				" thread_count=%" PRIuSIZE
+				" stwST=%d stwS=%d pass=%d",
+				__func__, max_threads_scantype,
+				curr_threads, thread_count,
+				stwST, stwS, pass
+			);
 		}
 # else
 #  ifdef HAVE_PTHREAD_TRYJOIN
@@ -1252,9 +1269,10 @@ nutscan_device_t * nutscan_scan_snmp(const char * start_ip, const char * stop_ip
 # ifdef HAVE_SEMAPHORE
 			/* Wait for all current scans to complete */
 			if (thread_array != NULL) {
-				upsdebugx (2, "%s: Running too many scanning threads, "
+				upsdebugx (2, "%s: Running too many scanning threads (%"
+					PRIuSIZE "), "
 					"waiting until older ones would finish",
-					__func__);
+					__func__, thread_count);
 				for (i = 0; i < thread_count ; i++) {
 					int ret;
 					if (!thread_array[i].active) {
diff --git a/tools/nut-scanner/scan_xml_http.c b/tools/nut-scanner/scan_xml_http.c
index c7db37c03c..934f66efd8 100644
--- a/tools/nut-scanner/scan_xml_http.c
+++ b/tools/nut-scanner/scan_xml_http.c
@@ -484,7 +484,11 @@ nutscan_device_t * nutscan_scan_xml_http_range(const char * start_ip, const char
 					__func__);
 				max_threads_scantype = UINT_MAX - 1;
 			}
-			sem_init(semaphore_scantype, 0, (unsigned int)max_threads_scantype);
+
+			upsdebugx(4, "%s: sem_init() for %" PRIuSIZE " threads", __func__, max_threads_scantype);
+			if (sem_init(semaphore_scantype, 0, (unsigned int)max_threads_scantype)) {
+				upsdebug_with_errno(4, "%s: sem_init() failed", __func__);
+			}
 		}
 # endif /* HAVE_SEMAPHORE */
 
@@ -510,11 +514,24 @@ nutscan_device_t * nutscan_scan_xml_http_range(const char * start_ip, const char
 				 * all earlier runners */
 				if (max_threads_scantype > 0)
 					sem_wait(semaphore_scantype);
-						sem_wait(semaphore);
-						pass = TRUE;
+				sem_wait(semaphore);
+				pass = TRUE;
 			} else {
-				pass = ((max_threads_scantype == 0 || sem_trywait(semaphore_scantype) == 0) &&
-					sem_trywait(semaphore) == 0);
+				/* If successful (the lock was acquired),
+				 * sem_wait() and sem_trywait() will return 0.
+				 * Otherwise, -1 is returned and errno is set,
+				 * and the state of the semaphore is unchanged.
+				 */
+				int	stwST = sem_trywait(semaphore_scantype), stwS = sem_trywait(semaphore);
+				pass = ((max_threads_scantype == 0 || stwST == 0) && stwS == 0);
+				upsdebugx(4, "%s: max_threads_scantype=%" PRIuSIZE
+					" curr_threads=%" PRIuSIZE
+					" thread_count=%" PRIuSIZE
+					" stwST=%d stwS=%d pass=%d",
+					__func__, max_threads_scantype,
+					curr_threads, thread_count,
+					stwST, stwS, pass
+				);
 			}
 # else
 #  ifdef HAVE_PTHREAD_TRYJOIN
@@ -643,9 +660,10 @@ nutscan_device_t * nutscan_scan_xml_http_range(const char * start_ip, const char
 # ifdef HAVE_SEMAPHORE
 				/* Wait for all current scans to complete */
 				if (thread_array != NULL) {
-					upsdebugx (2, "%s: Running too many scanning threads, "
+					upsdebugx (2, "%s: Running too many scanning threads (%"
+						PRIuSIZE "), "
 						"waiting until older ones would finish",
-						__func__);
+						__func__, thread_count);
 					for (i = 0; i < thread_count ; i++) {
 						int ret;
 						if (!thread_array[i].active) {