I hit this issue with an "H:" homedrive that is on a network share, and then in Windows is using "offline files" to keep a local copy. .idlerc was not cached so IDLE worked when online/connected to my work network but not when I was offline.
The temporary workaround was to mark .idlerc as "available offline" when connected. I wanted to document this in case others hit this scenario.
I also noticed that there is a .idlerc directory created in my local user directory c:\users\[username\.idlerc. This one has the recent-files.lst in it. So IDLE is creating two copies of .idlerc - one in the environment-variable-defined (and roaming) home directory, and one in the default local user directory. I would suggest that as this bug is investigated, you keep track of both instances of .idlerc.