The password/username's being rejected probably has something to do not installing bitcoin core in the default installation paths.
-snip-
I don't know why that worked-out for you
but I've tried to reproduce this by using Bitcoin Core installed on a non-default directory and Spectre connected to it just fine.
My best guess is it had something to do with your previous installation's setup
Like for example, if you haven't set a custom data directory in the welcome screen but set rpcuser and rpcpassword to the bitcoin.conf file in the custom datadir instead of the one in the default data dir.
But since its GUI settings are now overwritten by the reinstall, we don't have a way to confirm if that's the case.