After the recent update, all my extension suddenly disappeared. When I tried to install them again, it is not getting installed.
Version: 24.2.0.3 (X86_64) / LibreOffice Community
Build ID: da48488a73ddd66ea24cf16bbc4f7b9c08e9bea1
CPU threads: 16; OS: Linux 6.6; UI render: default; VCL: gtk3
Locale: en-US (en_US.UTF-8); UI: en-US
Flatpak
Calc: threaded
Hello!
I have the very same problem!
It did already happen a few weeks ago then, for some unknown reason, it worked. But today it does not!
I’ve tried moving the extension file around to be sure, double checked flatpak overrides (with flatseal) and so on.
I tried opening the extension file from the filemanager, through Libreoffice Open menu, and also from the extension menu, but none of this worked: I have the same error.
Any idea?
EDIT: I tried libreoffice flatpaks from both flathub and fedora repositories → same
I reply to myself with something which worked for me:
→ having the extension available over http, and opening the extension to install from the browser! 
Note: I tried with file:///home/slt/path/to/file.oxt but it did not work! I started a quick HTTP server on port 8083, open my browser to http://localhost:8083, navigated and clicked on the extension to install and it worked. Very weird!