Hi Michel,
Thanks for sharing that information.
I have done some tests, breaking the PackageContents file and these were the results.
1. If I break the file by writing syntax errors, for example deleting a <, then all add-ins fail.
2. If I break the file, with value errors (and syntax is correct), for example setting attributes SeriesMax and SeriesMin as "asd123", this addin is not added to Revit, but the rest works. And It is per version. I broke the Component block that sets the ad-in on Revit 2018 and it only failed on that one, all other 2019 and 2020 versions worked.
3. If some attributes have incorrect values, the add-in could work too, for example if i set SeriesMax with "asd123" but SeriesMin is ok, the add-in works. If the Version attribute is wrong, the add-in works as well.
So, the only scenario I found, where all add-in fail is when there is at least one syntax error in one of the PackageContents files. Possibly there are more reasons, besides this one.
What I would do next time, if it ever happens again, is drop one by one, each bundle folder until I get the one that is generating the error. If my guess is correct, there should be at least one PackageContents with a syntax error and after dropping it, the rest should work again.
Although from what many have described above, it seems that it is not enough to consider it as the only reason.
Kind regards !
Github:
https://github.com/franpossetto