Hi Basti,
Check whether the user defined to start MaxDB Services(database, database e service and xserver) has proper authorizations over the network share. By default these services are started using the Local SYSTEM account which seems to not have change permissions in network shares.
Also, check your network infrastructure and NAS(maybe event viewer can give you more info).
Meanwhile, to not break your backup cycle, it would be a good idea change the LOG backup template to generate the LOG backups in a local drive and schedule a user script to copy the LOG backup files to the network share meanwhile you verify this.