As I wrote it, the chat addon only displays received messages that have the correct message prefix, so something your end was whispering messages to everyone else via the addon channel, using that prefix.
The only bit of code that's common between this addon and others is the library that throttles chat, to keep you from getting disconnected when you send too much. That's ChatThrottleLib, which is also a part of the AceComm library, used by a lot of addons. That library puts all the data sent to it into a queue, so if the queue gets corrupted, one addon can end up getting information for another. The thing about F2PChat though, is it prints out all the data it gets, other addons might just ignore it if it doesn't look right.
When WoW loads the addons, it'll use the first copy of the ChatThrottleLib library it comes across, up until it finds a copy that's a newer version, so the problem could be a corrupted copy of the file from any recent addon, that for some reason is mixing up the data sent to it.
You could narrow down the possible problem addons by looking for ones that include ChatThrottleLib.lua in their folders (doing a windows search on your 'World of Warcraft\Interface\Addons' folder for that file name should find them all).
