Copying the 5.14.1 version over your newer version and then committing it again (as suggested by Roeland) will work but will also make your project history very confusing. If you want to solve this neatly you should install tortoiseSVN (make sure to install version 1.7 as Mendix is not compatible with newer versions). This will allow you to interact with the team server much in the same way Mendix does but will give you far more options. Among those options is reverting to an older version of your project. Simply reverting to a pre-5.14.1 version should solve all your problems.
Tortoise is a widely used product with lots of users; some simple googling should point you in the right direction on which steps to take.
Nikel's answer above was a good solution for my issue.
I created a branch line from the latest 5.14.1 revision on teamserver, downloaded it, and am now able to work in the 5.14.1 modeler and commit changes.
I don't perform these actions myself very often but you can try to copy the correct mpr (backup) over the one in your project folder. I just tried the revert merge changes on this situation, but that didn't work.
There is no simple ideal solution here. Mendix sets the modeler version used as part of commit metadata in SVN. Whenever the model is checked out, this is read by the modeler, if it does not match with the version in the .mpr it is likely the modeler will just crash. So just using Tortoise to commit your 5.14 version while the latest Modeller commit was done with 5.21 seems like a bad idea as this would result in this mismatch. You could overcome this by faking the commit metadata as well to say it was done with Modeler 5.14. I haven't tried this but it should be possible.
Another solution would be to just create a branch from your latest 5.14 commit. You could then keep on working fine with this branch, but you would need to take into account that you mainline has become useless and should not be used anymore. This is what I would go for in this case.
Nikel's solution - to create a branch - worked for me. Thank you, Nikel.