I recently tried updating from forge 436 to 445. When I did (and made no further changes to the server that was working prior) I got the following error upon attempting to login:
Once I downgraded to forge 436 again I had no problems. I then attempted to install a mod by dropping into the client/server mods folders, and recieved the same error. I really don't know where to take this except here.
Server forge log: https://gist.github.com/b43bdf1db0ae4c0c0ca5
Client forge log: https://gist.github.com/83c1ec80c18682c0f3b4
I've tried to find this error elsewhere and can't seem to do so. There are others with Internal Server Error, but not with the Error handling login related packet / Failed to handle packet errors.
I tried removing all JAR mods (except forge, ofc.) and the error still occurred. I can't think of any further information at this point. Let me know if I can be more helpful.