Continuing discussion with @stephdl
Does it work also with Apache ProxyPass?
Yes I think so, smeserver-backuppc is done in this way
a token to authentify with its own apache configuration file (on a different port) and a proxypass in apache to answer on the /BackupPC
So each Apache instance should be configured with a similar directive?
What about non-apache servers? Do you know if the protocol is supported?
Yes, I should read some docs
Well maybe something else must be used, I refer to https://linux.die.net/man/3/mod_auth_tkt and they talk of apache22 and before.....not apache24.