what do the sync settings look like? (see example screenshot. You get there via settings->accounts->account sync “kontensynchronisierung”)
Is the login proper? did you enable 2FA on the online account (neccesitates app passwords instead of the login password).
I’d make a .vcf backup export of the device-local contacts (via Contacts app settings) and re-setup the account after making sure credentials are proper (at the murena.io end). Worst case, reimport .vcf.
There’s also the possible case of contacts being partly device-local, outside the carddav synced account. Android has composite contacts, confusing.