I am unfamiliar with vTiger and have no idea why it would do that. My guesses are that either it uses some extrenal resource (likea javascript file or something) and it can't find it currently; or perhaps a security update has broken something (I would suggest that this is unlikely but not impossible).

Do you have a recent backup? Assuming that you do, perhaps restore to a new server (VM or cloud) and see if that too has the issue or if it works...