To me, it looks like it's trying to log me in automatically (which it shouldn't) and has the wrong user/password, but I admit total ignorance to MySQL. I was something I needed to click a few things for to set up the blog (on my website hosted by https://crosswinds.net/)
I don't really know anything about PHP either, Since I'm also unable to get to the administration side of my site, Crosswinds would be where to find what versions they are.
An unexpected error has occurred!
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Is it your hosting provider?
When I first installed B2Evolution, I killed my first site through softaculous (Or whatever) and installed B2. It immediately threw errors everywhere. Including the one above.
I killed b2 and waited 5 minutes (Ran out of beer) and tried again, it came up perfectly.
I did an upgrade thru the backend and it blew up my site. (was a third upgrade). playing around, I found out that if I cleared out /publc_html/_cache and then do a reinstall or upgrade, it ran fine.
Still waiting to hear from webhost as c-panel isn't letting me in either. They may clear up both problems ... maybe.
As it says in the message you have a problem accessing the database, so not a b2evo error. Let's hope your host can unblock your access quickly given you are probably using the correct use name and password.