During chat we tried to uninstall MU on staging site (details in chat transcript) with help of that tutorial https://premium.wpmudev.org/blog/uninstall-multisite/
After doing all the things site started to throwing error 500 internal server error.
First we tried to fix htaccess, but it didn't help.
After enabling debug log, site started to showing Fatal error on front end about the WPML plugin.
However - generating new debug log to see if there is something more there didn't work. No new file, despite fatal error still showing.
We tried to disable all the plugins by renaming plugins directory, but there was no effect.
This is staging site.