Have you tried specifying the full path specification for the file? It may be looking in the "current directory" and that might not be where the file is. The message "file not found" is unlikely to be a bug in SQL Anywhere.
What documentation are you using to guide you with sqlpp? The sqlpp -I (uppercase i) option is not documented and AFAIK never has been. In V5 there was a sqlpp -l (lowercase L) but that was for login userid and password.
The reason for that question is this: If the docs for some other vendor's utility "like sqlpp" are being used, it may be misleading in other, even more important, ways.