When trying to convert older AVI content to medium MP4 quality (having recently imported Sony Digital8 DV-AVI format using PMH import), at end of the MP4 conversion process the Conversion Results window displays, "An Unknown error occurred. (Error code: 32771)", immediately after which the generated MP4 is deleted.
process: Convert to MP4 and Save: Video Quality: 1920x1080 25p/30p 14 Mbps
Following the error, I restarted PMH and tried again - the same problem occurred again.
As a workaround, I used the opensource "VirtualDub" program, created a copy of the original AVI file (simply performing a "direct stream copy") of both video and audio. I then attempted to perform the conversion of the copied AVI to MP4 -- which completed successfully - no problems detected - resulting MP4 played fine.