Open PCManFM. Select Edit>Preferences. Click the Advanced tab. What do you have selected for Archiver integration?
I had xarchiver listed - which I see I don't have installed.
I must have inadvertently switched this when I was looking at the various tabs. I certainly don't remember doing it - but since it used to work and now that I have reset it to file-roller it is working again, I guess I did during one of my brain dead moments.
Thank you djohnston and Neal for providing quick replies.