Backup stuck

Hi,
The backup on our site is stuck on “Adding batch to zip file”.
I tried reducing the split file size to 300MB, still failed with the same log messages.
I tried reducing the split file size to 25MB, still failed with the same log messages.

Here is the log:

0000.001 (0) Opened log file at time: Wed, 24 Oct 2018 07:55:13 +0000 on https://www.socialfilmfestival.org
0000.005 (0) UpdraftPlus WordPress backup plugin (https://updraftplus.com): 1.15.2 WP: 4.9.8 PHP: 7.0.31 (cgi-fcgi, Linux de12.fcomet.com 2.6.32-896.16.1.lve1.4.54.el6.x86_64 #1 SMP Wed May 2 07:43:19 EDT 2018 x86_64) MySQL: 5.7.23 WPLANG: he_IL Server: Apache/2.4.34 (cPanel) OpenSSL/1.0.2p mod_bwlimited/1.4 Phusion_Passenger/5.1.12 safe_mode: 0 max_execution_time: 900 memory_limit: 256M (used: 55.8M | 58M) multisite: N openssl: OpenSSL 1.0.2k-fips  26 Jan 2017 mcrypt: Y LANG:  ZipArchive::addFile: Y
0000.082 (0) Free space on disk containing Updraft's temporary directory: 339012 MB
0000.085 (0) Tasks: Backup files: 1 (schedule: manual) Backup DB: 1 (schedule: manual)
0000.088 (0) Requesting semaphore lock (fd) (apparently not via scheduler)
0000.089 (0) Semaphore (fd, ff_options) was stuck, set lock time to 2018-10-24 07:55:13
0000.096 (0) Semaphore (fd, ff_options) reset to 1
0000.098 (0) Set semaphore last lock (fd) time to 2018-10-24 07:55:13
0000.099 (0) Semaphore lock (fd) complete
0000.102 (0) Backup run: resumption=0, nonce=b9a5ba76a877, begun at=1540367713 (0s ago), job type=backup
0000.103 (0) Scheduling a resumption (1) after 300 seconds (1540368013) in case this run gets aborted
0000.114 (0) Checking if we have a zip executable available
0000.116 (0) Testing: /usr/bin/zip
0000.128 (0) Output: zip warning: binziptest/test.zip not found or empty
0000.129 (0) Output: adding: binziptest/subdir1/	(in=0) (out=0) (stored 0%)
0000.130 (0) Output: adding: binziptest/subdir1/subdir2/	(in=0) (out=0) (stored 0%)
0000.131 (0) Output: adding: binziptest/subdir1/subdir2/test.html	(in=131) (out=107) (deflated 18%)
0000.132 (0) Output: total bytes=131, compressed=107 -> 18% savings
0000.148 (0) Output: adding: binziptest/subdir1/subdir2/test2.html	(in=138) (out=113) (deflated 18%)
0000.150 (0) Output: total bytes=269, compressed=220 -> 18% savings
0000.164 (0) Working binary zip found: /usr/bin/zip
0000.167 (0) Zip engine: found/will use a binary zip: /usr/bin/zip
0000.168 (0) Creation of backups of directories: beginning
0000.172 (0) Beginning creation of dump of plugins (split every: 25 MB)
0000.376 (0) Total entities for the zip file: 918 directories, 5283 files (0 skipped as non-modified), 77.3 MB
0000.380 (0) Zip: backup_2018-10-24-1055_wwwsocialfilmfestivalorg_b9a5ba76a877-plugins.zip.tmp: 100 files added (on-disk size: 0 KB)
0000.383 (0) Zip: backup_2018-10-24-1055_wwwsocialfilmfestivalorg_b9a5ba76a877-plugins.zip.tmp: 200 files added (on-disk size: 0 KB)
0000.386 (0) Zip: backup_2018-10-24-1055_wwwsocialfilmfestivalorg_b9a5ba76a877-plugins.zip.tmp: 300 files added (on-disk size: 0 KB)
0000.390 (0) Zip: backup_2018-10-24-1055_wwwsocialfilmfestivalorg_b9a5ba76a877-plugins.zip.tmp: 400 files added (on-disk size: 0 KB)
0000.397 (0) Zip: backup_2018-10-24-1055_wwwsocialfilmfestivalorg_b9a5ba76a877-plugins.zip.tmp: 500 files added (on-disk size: 0 KB)
0000.400 (0) Adding batch to zip file (UpdraftPlus_BinZip): over 500 files added on this batch (2.4 MB, 5283 files batched, 501 (501) added so far); re-opening (prior size: 0 KB)

What could be the cause for this?