monit: action failed -- Other action already in progress -- please try again later
Few moments later:
Checking all RRD files. Please wait ...
All RRD database files are valid.
Graphs in System Information/Status are good. But when manually refresh any graph - same error as above (hang on ERROR: No DS called 'value' in '/var/lib/rrdcached/db//localhost/nut-ups/percent-charge.rrd')
You can see that the DS name (data source name) has been changed from "percent" to "value". During upgrade those files were not wiped and recreated, so after upgrade OMV 1.x continues to use those old files. When omv-mkgraph of OMV 1.x tries to create graphs for NUT, it looks for DS name "value" and can't find it when processing rrd files.
The solution is, as ryecoaaron mentioned, to remove directory /var/lib/rrdcached/db/localhost/nut-ups/ after uninstalling and then reinstall openmediavault-nut. Preferably the uninstall script should remove them for users.