Some customers report errors to do with missing default avatars. These might appear in your error log with messages like "failed to open stream: No such file or directory" with a URL for something like "file.php/avatar/25/default.jpg".
This happens when the default avatar image has gone missing on disk. How this happens can't be known (i.e. someone deleted the image, a server move gone wrong, etc. the possibilities are endless). But since these default images are often used, their absence causes the error log to fill up quickly.
In this specific case, it's simple to restore these images by executing a MySQL query manually:
DELETE FROM blobs WHERE sys_name IN ('picture-default', 'orgpicture-default', 'picture-default-agent', 'picture-default-team', 'picture-default-dep');
This will cause the system to re-insert the missing file next time it's requested.
Note that this only fixes this specific missing file. If this file is missing, it may be a sign that other files are missing too. It's a good idea to get your sysadmin to verify that the disk Deskpro is saving files to is in good health.