It was not intended to be rude, but I'm only human and have my bad days as well.
There are no rules for the size or so, but there are now rules for the naming.
MV uses networking specifications for filenames, and therefore a lot of special signs and non-latin letters that were tolerated from Windows (and therefore by Ace) are now absolutely forbidden. The MV engine cannot read files whose names do not conform to networking specifications. That means %, space, öäü, accents, non-western alphabets - all those were allowed in filenames under Ace (which conformed to Windows) but break MV (which needs networking rules for all files due to deployment to non-windows systems).
Additionally A and a (capitalisation) means different letters under networking rules - Windows loaded a file named "Abc" even if the program string contained "abc" - under networking rules that will fail.
Those are some possibilities I couldn't check in the video because the filenames were unreadable small.
There are several other things that could have caused this (for example sometimes you need to reload the entire project before a file is detected that was moved into the project while the project was still open in the editor, because the update to recognize folder changes isn't always reliable - but in those cases the file will be there after a reload)