Sigh, I just wrote a lengthy post including redacted screenshots, only to realize that when I got to the point where it didn't work yesterday, it suddenly does
Just for completeness, when I opened an extension with n+1 MAC addresses for MAC based provisioning and accessed https://vpbx.mydomain.com/prov/snom370-0004132E0000.htm, that only worked for the first MAC address - now it works for all of them again. Thanks for the response anyway!