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Are you using XOOPS Protector? If so it may be the cause.
See this thread for some more info. If that's not it, I can't help.
In my case, I have no idea what triggered the error, however another CMS that I run on the same host suddenly refused to allow a login on around the same day. This was ultimately attributed to a date/time discrepancy between my host's website server and the DB server.
I ran a test to check the web server date/time (which was an hour slow) but I don't know how to check the DB server.
Maybe the battery ran out and it's 1984 again.
A thread is for life. Not just for Christmas.