Error is written only while JavaScript verification are passed, but server verification not, but it's impossible if you do not change our JavaScript.
I'm not running anything that is able to affect the JavaScript on your site, no.
Could it be that your server verification is stricter than the client verification?
The password I tried setting was 9AD#Npj@9L7vKK&VlzaIrDS%Z7X7E5WN - could it be any of the "special characters" that broke things?
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Yes, apparently it's the & in the middle.
Setting the password to "abcdefghijklmn&mnopqrstuvwxyz" gives "Error" but removing any of the letters makes it legal. It seems the bug only happens if there are at least 14 characters either side of the ampersand. I can't imagine what you're doing that would cause this to happen.
Edit: now I'm using the 'dooglus' account that Arrogant registered for me. I'm trying to change the password, but each time I go to settings, type the old password once and new password twice, then "submit" it, it tells me "Email has been successfully changed" and doesn't change the password.