Just do what it says.. Reboot the thing and keep the CTRL key pressed untill the system in fully booted.
This will clear up some extra memory, giving some more room which the software probably needs.
The msx1mania loader is very picky on available ram, probably the ocm requires more workspace. You could try converting them to rom and use the rom emulation of slot-2 of the ocm.
Converting the dsk images can be done with dsk2rom. Loading them, havn't tried rom images on my ocm yet, but if mgload is the tool for that, use that one dsk2rom allows creation of scc mapped megaroms, so that shouldn't be a problem.