It looks like the following errors are cause of the image resizing problem:

[08-Jan-2019 14:10:28 UTC] PHP Fatal error:  Maximum execution time of 30 seconds exceeded in /home/.../public_html/plugins/intervention-image/Intervention/Image/Commands/Argument.php on line 130
[08-Jan-2019 14:11:42 UTC] PHP Fatal error:  Maximum execution time of 30 seconds exceeded in /home/.../public_html/plugins/intervention-image/Intervention/Image/AbstractDriver.php on line 105
[15-Jan-2019 14:25:09 UTC] PHP Fatal error:  Maximum execution time of 30 seconds exceeded in /home/.../public_html/plugins/intervention-image/Intervention/Image/Commands/Argument.php on line 40

I can't determine what causing these errors, maybe your hosting support can help.