Just noticed an error message after updating to 4.2.2.

[kh99]

If you can't tell by looking at the list of plugins, then you can try temporarily disabling your products one by one. Obviously if you have one that has to do with login failures you'd start with that one.

If you have plugins that aren't part of a product, then look under the "vbulletin" section in the pugin manager for one that uses that hook.