I had the same issue as ndarkstar. Login works properly, but after that, the only available realm "MMOPro.org" says "(incompatible)" whilst both server and client are running version 6.2.4_21355. Client gives error code: WOW51900225.

The incompatible realm thing got me thinking it should be the realm settings which are in the database. I used the HeidiSQL editor to look at the realm configuration: database 'auth', table 'realmlist'. The only available record ('MMOPro.org') had gamebuild set to 20726. Obviously it should be 21355.

The following query can be used to set all realms to gamebuild 21355
Code:
update auth.realmlist set gamebuild=21355 where gamebuild<>21355
After that login worked properly for me!



Hope that helps!