well I am not at all familiar with your version of vbgallery, so help is difficult.
but at first sight.. there are 3 possibilities.
A. you got hacked (I do not think so.. as there would be more problems than that)
B. For some reason the file register.php got deleted.
things like that happen.. even on a normal computer that doesnt run 24h/24h after some time the harddisk or something else starts to get errors.
C. Your provider upgraded PHP... or added some Patch giving internal errors.
Now personnally I do not understand how you could be running vb
3.54 on a normal server. because I had problems running vb
3.7 on my server. too many errors could not install. vbgallery 2.2 can not be installed either under php 5.34
But I suspect C to be the cause.
or B where it can be any file needed by the system.
Try to find your last backup and restore at least the files.