I created the project with R 3.44 (was the newest R version on rstudio cloud at that time). However, recently when other users used the link above and thus created a temporary copy of the project, the default version was the newest R version 3.5. (When I logged in myself, it was still R 3.4.4) Unfortunately, this had the effect that none of the packages that I had manually installed in my original cloud project was available by default in copies of the project and the project could not be used as expected.
I could fix the problem by changing my original project to R 3.5 and installing all packages again. Yet, at least for some time my project was broken.
I am not sure whether that behavior is desired or a bug. Personally, I think it would be much preferable, if a copy of a project, would by default have the same R version as the original project to guarantee that projects can be copied and used without errors. If there are reasons against making this the default behaviour, it would be at least great if one could set this behavior for copies by an option.