Someone else reported that this can happen if they try to like / thank a post from a user that has been deleted.
Sadly without any common denominators I can't do much
I'll put up a change later today that will print out the postid it thinks is invalid, that should help narrow it down
EDIT: Grab a Lite version with this change included here:
http://www.dragonbyte-tech.com/vbeco...o&productid=22
Let me know when you've found something in common with the postids that produce the error (specific usergroup, deleted users, etc) so that I can test it
Fillip