Language pack build cannot be installed, "package invalid" ("Paket ungültig")

[Cyberman-tM]

I've tried to update my klingon language pack to the latest(?) build (taken from here: https://github.com/AnySoftKeyboard/AnySoftKeyboard/tree/release-branch-addons-v5.0-r1 ) .
After a bit of work I got it to compile, but when trying to test it on my phone I get an error that the apk cannot be installed.
I've enabled developer mode and Files (by google) is trusted to install APKs.

It's probably a minor error in the configuration somewhere, but I'm hoping someone recognizes the problem and has a suggestion.

Steps to reproduce

1. create a new language pack
2. compile it to apk (release)
3. copy to phone
4. install apk

Expected behaviour

APK with language pack should be installed

Actual behaviour

Getting an error, package invalid/cannot install. (German: "Paket offenbar ungültig)

Android OS version:
13

Device manufacturer and model:
Ulefone Armor X13

List of installed add-ons (like languages, or themes):
only ASK itself

[menny]

Can you share your latest code? A branch on your fork, maybe?

[Cyberman-tM]

If I did it right, this should contain the latest changes I did in the current addon-branch:

https://github.com/Cyberman-tM/AnySoftKeyboard-Klingon

[Cyberman-tM]

Did you have time to check it? Where did I go wrong?