I have set DevonThink to automatically OCR scanned documents. I have just scanned one document where the first page was handwritten which resulted in a filename with nonsense and including special characters. On that file, the OCR failed with an error message to the result that the file could not be opened or found (unfortunately I do not really recall). It also failed with the same error when I asked it manually to do OCR.
Then I renamed the file (via Get Info), and with a more sensible filename it worked. So it was really the filename that was the issue.
Here is the filename