The current testing release 3.9 which is about to become the stable 3.10 one day gets more and more problems fixed. Main improvements are:
- switch to Qt6 on most platforms
- updated Centreon
- improved Zabbix
- Icinga2 fixes
- Checkmk 2.0 fixes
- and many more small fixes
Have a look at https://github.com/HenriWahl/Nagstamon/releases/tag/latest to help finding more bugs. Even better - check the issue tracker at https://github.com/HenriWahl/Nagstamon/issues to help fixing the already found ones.