As far as I can tell from some searching on Japanese websites, error 40 seems like something that can happen when the disk is somehow corrupted.
Enri's page says "一括ロードでロード出来なかったファイルがあった(ファイル数が足りない)", which essentially translates to "A "batch" file load operation failed to load one of the files (there are not enough files [on the disk?])".
Then
this page has a scan where it says "ディスクカードを取り出し、本体の電源を1度切って、最初からロードし直しましょう", which translates to "Take out the disk, power off the console and try again from the start." (e.g just instructions on what to try, rather than a description of the problem).
It's possible that the FDS file you're using is corrupted or was dumped from a corrupted disk.
It could also be something that relies on a specific FDS behavior that is not properly emulated by any of the current emulators, too.