well, I don't know what to say. first off, thanks for the replies.
to my issue, I spent all day today opening and closing VW folders and files. I thought I kept VW 20 on my computer so I was anxious to open that version to see if I would have the same result, but I must have deleted it. I had copied old files to a thumb drive and when I opened files and saved back to that drive, all worked fine. this led to other trials—many trials, too many to keep track of so I cannot give any advice that would make any sense and might just confuse. I 'cleaned out' folders containing the same files but with different versions of VW. I opened various files stored on my iMac and on my 'time machine' external Samsung SSD drive, I made simple modifications, and used 'save' with new file names (I also did this when I started having the problem just yesterday, but with no luck).
to make a long story short: I am no longer having this 'Internal Error...' problem. it has disappeared as quickly and as mysteriously as it first appeared. I'm guessing i must have rearranged file paths while moving folders and files and the problem self-corrected. I know this won't satisfy anyone looking for a solution, and I can only hope others experiencing a similar error message will have similar luck.
FYI: I am using iMac, v11.1, VW 21, SP2. I hope my luck keeps up. I may come back again!