Try going to the registry, as well as deleting the folder in program files.
Here search for the keys "MGS" and "Microgaming". These will have the subkey "casinos", and then keys for each MG casino. delete the key "royalvegas" in both of these, and no others, as ALL MG casinos would then stop working.
Set a restore point BEFORE doing this registry action, but AFTER uninstalling and removing the folder from program files.
This should have removed ALL retained references to Royal Vegas.
The problem with this particular casino is that it was converted to auto-login when they changed to the new lobby, and this would prevent you from correcting a corruption in the login fields.
There is an alternative, less drastic, that might work after you get the first failed login message. Go to options, and uncheck "automatic login", and try again. Then overtype your account number and saved password. This might correct any corruption present in the registry for these fields, and is safer than the registry edit option.
I have had to go this far in the past with corrupted MG casinos, and the registry cleanup & folder delete DOES indeed work when all else seems to fail.
To be absolutely sure, switch off the PC after this, and get a clean boot before reinstalling Royal Vegas (I am not talking about footwear here

).
If this is about MPV, these are now also available in Platinum Play, so you could still play the free one for the big event there if this remains unresolved.