Hi guys,
I actually figured it out. This is how I did it, maybe this will help others:
1. Updated themes
2. Network disable themes
3. Network disable plugins
4. Disabled main site plugins
5. Increased output_buffering from 4096 to 8192 in /etc/php.ini
6. Installed new wp multi-network manually by unpacking (does not really clash with old version)
7. Upgraded WP
8. Removed old wp multi-network (manually by deleting old file)
9. Started to uprade plugins one by one: Some are no longer supported in WP 4, others claim they are but are not working – so I advise to backup, enable and test and then another one…
(I have the feeling my manual install/removal was actually not necessary, but I’m not going to back to proove that – you can try to uninstall old before WP upgrade and then install new…)
Conclusion: WP-multi-network is working fine in WP 4.4.2, thank you very much for your great plugin !!!