Tvheadend

The structure of the adapters/muxes/services has changed significantly - that's what the migration should handle. I can only guess why it didn't work (perhaps it's because you'd already started the new version, perhaps it's because you're running from command line and don't have the right permissions?), but I'd suggest that adding muxes and re-scanning is the safest way of getting your channels back.

While I've never had the experience you're describing, I have deleted everything and started fresh many times, it doesn't take long to rebuild everything. The hardest part is re-creating all your DVR rules, and you can even view that as an opportunity to have a clear out of the stale ones!

You didn't say from where you installed or how, by the way, or why you're running from console versus as a service. Something to think about.

The error in script is in line "path = args⁰".
Don't remember what was exactly written there.

Adding muxes and re-scanning is too painfull, as I've sorted in groups by number about 1K channels from 6 sats.

I installed everything (unstable 3.9.* when upgrading and stable 3.4.* when downgrading) by changing the rep and by "apt-get install tvheadend".
After every installation I did "sudo chown -R ..." for the folders.

What is more strange - why after downgrading (delete 3.9.* and install 3.4.*) nothing works:
a) 3.5.* (subfolders are in the share) > 3.9.*(subfolders are in the share) > nothing works > 3.4.* (subfolders are in the share),
b) full delete 3.4.* (in synaptic and by deleting subfolders) > 3.4.* (fresh install) > stop > (copying subfolders from backup) > restart > nothing works

The architecture of 3.5.* (backup) and 3.4.* (fresh install when downgrading) is almost the same.
GUI is blank with no my choices (channels, groups, etc.)

Answering your question.
I use console to kill tvheadend 3.4.*, copy folders from backup and start it again.

I've never seen a need to chown if you're installing properly. The tvheadend service should run automatically as hts and thus have full rights to /home/hts/.hts/tvheadend.

I may be barking up the wrong tree, but check the UID/GID under which tvh is running, and check that it has access to the config files/directories and adapters. Something's clearly not right here: if nothing else, if you're restoring from backup (and keeping the ownership and permissions correct when you copy) then 3.4/3.5 shoudl work fine with 3.4/3.5 config files. It would be true that 3.9.x wouldn't understand 3.4/3.5 configs, though, as likely as not - too much has changed.

I found the fault.
It is in difference in folders between 3.5 and 3.4 in my case:

/home/hts/.hts/
/home/[pc_user]/.hts/

/usr/bin/
/usr/local/bin/

/usr/share/
usr/local/share/

And also in rights: hts vs. [pc_user]

If I uncheck "/usr/bin/tvheadend" to make Ubuntu start TVH 3.4 on PC starting, TVH works with /home/hts/.hts/
If the option is checked, TVH 3.4 tries to use /home/[pc_user]/.hts/

So I unchecked the option and copied all backup files from /home/[pc_user]/.hts/ to /home/hts/.hts/, changed rights to "hts" and it's OK.