Thx!! I did as it was described in ‘Solution’ part.
It’s now (partly) fine but something looks odd:
when trying to use “UPDATE SYSTEM” from ‘Welcome box’, I got message:
“Database synchronisation didn’t succeed” (just one warning)
when trying to update via terminal, it says it’s OK:
sudo pacman -Syu
:: Synchronizowanie baz danych z pakietami…
maboxlinux is up to date
core is up to date
extra is up to date
multilib is up to date
:: Rozpoczynanie pełnej aktualizacji systemu…
nie ma nic do zrobienia