Low hanging fruit:
The error message is just an example - maybe not the best one. But in this case I'm pretty sure, I didn't reject anything, but simply wanted to log in. The point is, that on almost every error message I have to confirm, that "Ok, I understand", which in 95% is not the case. This makes people angry, maybe on an rather uncautios level and just again results in a bad perceived UX. Please change that! A really low hanging fruit. Don't make people angry for no reason.
Again: they have to actively agree on something, they really don't agree on. THis causes frustration additionally to often being not able to proceed.
A "x" in the upper right hand side corner (as usual) will do - why a button? It is still not satisfying, as we have an error, but I'm not unwillingly forced to agree on something.