If Reaper and it's user-settings is the problem then try on a fresh portable with default settings, can't hurt to try and sometimes that is just easyer than trying to spend time finding a needle in a, when one has customized alot, tweaked.
And you can save\load your Reaper settings when ever.
Ps, I need to do that one myself.. schhh
The short version, try default'ness and as new fresh you can incl new project so it's not about routing plugs or what ever, new, simple as possible.