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Anything that pops asking for your username and password in a scam, yes. Never provide them to anything other than the legit Gaia login box.
ALTHOUGH; if the servers are busy and running slow, there is a legitimate error message that gives the 'Slow down buddy' error message- it does not, however, ask you for any personal information at all. You just need to reload the page to clear it after waiting a few moments. I've seen it a few times, mostly during events. It basically says 'Slow down buddy, you're loading pages too fast. Take a few minutes to cool down' and then nothing else comes of it.
The error you received may have been legit, but the popup associated with it certainly was not. It could also have been a clever scriptor using one of the less common errors in an attempt to make the message appear legit.
As said above, report the incident and page with any information you have available.
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