Errrrr... I can't reproduce it now :(
Maybe I should be happy it's gone but I have a hard time accepting the fact that there was an error which I have no idea what caused it.
I am 100% sure it wasn't a connectivity issue, as the error occurred ONLY in the main site.
The 404 was a blank page with some text. Nothing WordPressy. Should have print screen it.
Hopefully I will get it again and upload the info. Yeah.. I just wished for an error..