I had the same issue a few years ago.
It was a Varta 625U causing the problem. Varta had changed the form factor of the collar around the battery core ever so slightly, resulting in the battery not fitting in the M5 anymore.
A fantastic Leica technician here in Germany produced a new camera cover for me that fits SR44 batteries.
I wouldn’t now how else to work around this issue.
Hope this helps.
Best,
Johnny