I have a ASTROMETA DVB-T2/C dongle, i use both tuners whit Prog DVB, having a memorized channel list for
terestrial and cable channels, untill now everythings Ok, but always when i open the Prog DVB again for watching the DVB channels, when i look
to the multiplex list again, the base is empty, and if i want to make scan , whatever to DVB-T2 or DVB-C, im not able anymore because the software blank all the
multiplexes list and nothing to do untill a new install..so weird..

Is it last version? Not good

is 72,26,03

What folder of ProgDVB installation? Check Transponder\CabFiles folder.

there is the all.list cotaining all transponders (if i open whit notepad++ im able to see)

x32 or x64? What folder of ProgDVB installation? Still not can repeat problem.

ps: try disable UAC or/and start app with Admin rights. Still same?

Try prerelease http://www.progdvb.com/download_progdvb_prerelease.html

I tested with version 7.26.3, and if I run as admin, it works ok. So the problem was some permissions on install directory.
Before, the transponders were in ProgramData and there was no problem in that location. Also, you could create a UserList file with custom transponders. Did this function changed? Or you can still use custom transponder list?