I've dug a little bit more and found an answer: the issue seems to be a logical bug in your project in PDFMerger.php near
throw new exception("Error outputting PDF to '$outputmode'.");
You test the return values of fpdi->Output which according to some documentation i found is a void function, so it's always false and throws an Exception all the times.
I'm manually fixing it by removing/commenting the code but i suggest you to fix it in the main branch for everyone to get it working properly.
Waiting for feedback, hope to have been of some help.