I can't recreate this so it's just an FYI. I clicked F7 to view a .eml file.About 1 minute later when I tried to close the viewer window the lister went into a Not Responding state. I left it for a while but it never came back to life so I did a DOpusRT /restart. Everything looked OK after the restart but when I attempted to rename the file in question I was told it was in use by another process which turned out to be DocsVW64.exe. After a Windows reboot all was well. Opus version is v12.4.3.
Sounds like the eml preview handler locked up. Probably outside of our control.
If a viewer hosted inside docsvw64.exe locks up then killing the docsvw64.exe process should be safe, and will usually let the lister recover. The process isolates the rest of Opus from the viewer, since 3rd party viewer components are not always stable.
Just found this thread as I was having the same issue with Illustrator files, hoping for a solution!
Currently, I just use IOBit Unlocker to unlock the file. Only other option is to disable the AI file from the "ActiveX + Preview + Office + Web" viewer plugin. Shame really.
The problem is likely with the AI viewer and not the process hosting it or any part of Opus. Not a lot we can do about that, except suggest reporting the bug to the company that makes the viewer, or looking for an alternative viewer for the format.
Killing docsvw64.exe is better than using an unlocker, if it won't exit by itself after the viewer pane has closed.
Actually, I found a little workaround... I disabled the AI file as mentioned and added this plugin you mentioned some time ago:
Seems to be working so far...