Network time machine and Big Sur

Macbook Pro (Big Sur) backing up to a networked disk designed as Timemachine attached to an iMac (Catalina). Worked flawlessly. iMac replaced by Mac Mini M1 (Big Sur). Attached newly formatted disk designed as network Timachine. No go. Macbook Pro can’t even see the disk. SMB seems broken between the two machines, both running Big Sur. Is there someting wrong with SMB between two Big Sur machines ?