I think you're claiming wrong stuff here. AGPLv3 section 7 paragraph b) expressively authorize the author to require an attribution in the derived work or copy. What Nextcloud did was to remove this attribution, so they actually mooted their own right to use the code under that license. There's nothing related to trademark or branding violation here. If OnlyOffice attacked Nextcloud for using their TM or brand for respecting the license, they would be debunked at a trial (if it even reach a trial), since they expressively allowed the use of the attribution in distributing their work with this license. Note: This license doesn't give you the right to use the branding of OnlyOffice on a derived product and claim it's yours or you're acting as them, that's a complete different usage case here.

You're right, the branding claim here is BS but attribution requirements are legit. I only took a cursory glance at their repo, but I don't see any copyright notices for OnlyOffice in EuroOffice. There should be.