I have some fixes in XoopsFormRendererBootstrap5.php for Xoops 2.5.11, but GitHub says "No more forks can be created." when I try to fork XoopsCore25 in order to create a pull request. Do you want me to contribute these changes some other way?

You can't fork it because you already have a fork of a fork (i.e. from me) - see the background here.

You have few choices:

1) the simplest one - you can delete my fork, and then fork the original XoopsCore25. Since you have access to all repositories in the chain of forks, you can set locally separate branches for different forks, work on a particular branch, and then make a PR to that repository (see how the author of the above question solved it).

2) There is also a more complex solution, if you want to keep separate forks, see: Create Multiple Forks of a GitHub Repo

I would recommend the option 1