Every now & then I get a message pop-up "Backup Detected" due to date conflicts …and given a choice between keeping the local file or allowing the sync to go forward.

The pathname of the offending file is not given, so I can't make an informed decision as how to act.
### Server configuration
Operating system:
Web server:
Linux DietPi 4.4.13-v7+ #894 SMP Mon Jun 13 13:13:27 BST 2016 armv7l GNU/Linux
Database:
Mysql
PHP version:
PHP 5.6.24-0+deb8u1 (cli)
ownCloud version:
8.1.9
Storage backend:
### Client configuration
Client version:
Version 2.2.2 (build 6192)
Operating system:
Window 10 64 bit
OS language:
Installation path of client:
C:\Program Files (x86)\ownCloud>
### Logs
Please use Gist (https://gist.github.com/) or a similar code paster for longer
logs.
`Template for output < 10 lines`
1. Output of `owncloud --logwindow` or `owncloud --logfile log.txt`
(On Windows using `cmd.exe`, you might need to first `cd` into the ownCloud directory)
(See also http://doc.owncloud.org/desktop/2.2/troubleshooting.html#client-logfile )
No ouput
1. Web server error log:
/var/log/apache2/error.log is empty
2. ownCloud log (data/owncloud.log):
{"reqId":"Q\/jga\/a221ql6y7bMiMN","remoteAddr":"79.177.220.46","app":"core","message":"imagecreatefrombmp: imagecreatetruecolor failed for file \"\/var\/www\/owncloud\/data\/henryd\/files\/Photos\/mumpics\/Scan.BMP\" with dimensions 466x-663","level":2,"time":"2016-08-07T04:44:46+00:00"}
{"reqId":"R8d5qBKgi0HzBFI9OWch","remoteAddr":"79.177.220.46","app":"PHP","message":"imagecreatetruecolor(): Invalid image dimensions at \/var\/www\/owncloud\/lib\/private\/image.php#700","level":3,"time":"2016-08-07T04:44:47+00:00"}
{"reqId":"R8d5qBKgi0HzBFI9OWch","remoteAddr":"79.177.220.46","app":"core","message":"imagecreatefrombmp: imagecreatetruecolor failed for file \"\/var\/www\/owncloud\/data\/henryd\/files\/Photos\/mumpics\/Scan-042.BMP\" with dimensions 1178x-616","level":2,"time":"2016-08-07T04:44:47+00:00"}
{"reqId":"HwXCb7bqcKxzmHHSSkFR","remoteAddr":"79.177.220.46","app":"PHP","message":"imagecreatetruecolor(): Invalid image dimensions at \/var\/www\/owncloud\/lib\/private\/image.php#700","level":3,"time":"2016-08-07T04:44:48+00:00"}
{"reqId":"HwXCb7bqcKxzmHHSSkFR","remoteAddr":"79.177.220.46","app":"core","message":"imagecreatefrombmp: imagecreatetruecolor failed for file \"\/var\/www\/owncloud\/data\/henryd\/files\/Photos\/mumpics\/Scan-001.BMP\" with dimensions 861x-1311","level":2,"time":"2016-08-07T04:44:49+00:00"}
{"reqId":"KYLJfk2uhrToKrCKeG6M","remoteAddr":"79.177.220.46","app":"PHP","message":"imagecreatetruecolor(): Invalid image dimensions at \/var\/www\/owncloud\/lib\/private\/image.php#700","level":3,"time":"2016-08-07T04:44:49+00:00"}
{"reqId":"KYLJfk2uhrToKrCKeG6M","remoteAddr":"79.177.220.46","app":"core","message":"imagecreatefrombmp: imagecreatetruecolor failed for file \"\/var\/www\/owncloud\/data\/henryd\/files\/Photos\/mumpics\/4\/Scan-045.BMP\" with dimensions 680x-979","level":2,"time":"2016-08-07T04:44:49+00:00"}
{"reqId":"3EMWGdm7yz1p5ujcuCPD","remoteAddr":"79.177.220.46","app":"PHP","message":"imagecreatetruecolor(): Invalid image dimensions at \/var\/www\/owncloud\/lib\/private\/image.php#700","level":3,"time":"2016-08-07T04:44:50+00:00"}
{"reqId":"3EMWGdm7yz1p5ujcuCPD","remoteAddr":"79.177.220.46","app":"core","message":"imagecreatefrombmp: imagecreatetruecolor failed for file \"\/var\/www\/owncloud\/data\/henryd\/files\/Photos\/mumpics\/4\/Scan-015.BMP\" with dimensions 774x-1105","level":2,"time":"2016-08-07T04:44:50+00:00"}
{"reqId":"jLMCQ\/gcvFT0toiJDN+F","remoteAddr":"79.177.220.46","app":"PHP","message":"imagecreatetruecolor(): Invalid image dimensions at \/var\/www\/owncloud\/lib\/private\/image.php#700","level":3,"time":"2016-08-07T04:45:01+00:00"}
{"reqId":"jLMCQ\/gcvFT0toiJDN+F","remoteAddr":"79.177.220.46","app":"core","message":"imagecreatefrombmp: imagecreatetruecolor failed for file \"\/var\/www\/owncloud\/data\/henryd\/files\/Photos\/mumpics\/3\/Scan-001.BMP\" with dimensions 901x-568","level":2,"time":"2016-08-07T04:45:01+00:00"}