Try a new session, first loading an instrument known to work,
followed by DecentSampler on a second track. If the first one
still functions after Decent quits, you may have narrowed the
You can also install an older Reaper in a different location,
or create a reaper ‘portable install’, (all in one folder)
to further investigate, or use without the problem.
Did you have any recent background OS update
in the timeframe of the issue?