I think this is a bug with your OS

how can it be resolved?? Do you have any idea?

It could also just be the case that the folder didn't exist yet?

it opens this on windows... I think this is a flatpak permissions issue..

might need to make it have access to ~/.temp/ or something.. not sure where mc does it on linux, but should be easy to test.

Does xdg-open open the folder? Make sure your XDG settings are correct.

I have the same problem. ArchLinux, latest PolyMC release from AUR (built from source) (I also tried the flatpak version with the same result), no "open directory" buttons work. No problems with other programs, xdg-open works fine.
It looks like Prism has the same problem and tries to fix it (at least for non-flatpak users) by removing the legacy MultiMC indirectOpen workaround in launcher/DesktopServices.cpp completely (see PR 1954 from their repo).
It looks like this workaround is really obsolete: I can't find it in other popular programs, like QBittorrent.

P.S. Flatpak support can be a pain: in my project I had to implement DBus support (org.freedesktop.portal.Desktop, OpenDirectory method) to open folders from a flatpak installation with correct mappings to host filesystem. The problem is OpenURI XDG-Portal method (seems to be used by QT) does not support file:// schema, you required to use OpenFile/OpenDirectory methods

Interesting. I'm very busy but i'll take a gander at it within the next week or so.

I have compiled PolyMC with the removed indirectOpen workaround and can confirm that it fixes the "Open Folder" buttons for non-flatpak installations.

I tried searching QT sources and found that it uses the OpenURL and OpenFile XDG Portal methods but doesn't care about OpenDirectory, but you can use their implementation of the OpenFile call as a reference

that it fixes the "Open Folder" buttons for non-flatpak installations.

I'd much rather find a fix that works for both flatpak and non-flatpak installations.

The only way to support Flatpak is to implement the OpenDirectory call by yourself (or find an external dependency?) with the QDesktopServices::openUrl call as a fallback for non-DBus non-Flatpak installations.

Good news: I was wrong about Flatpak support. This patch works for Flatpak installations as well.

I can spend some time, apply the full patch from the Prism repo and submit here as a PR
You can check the patch in my fork

Awesome. Open a PR here and I and others will review it.