You can run into this issue with any two sync programs that operate on virtual files, as another commenter said. This isn’t specifically a OneDrive or NextCloud problem. You can safely run both at the same time on the same machine, as long as they are syncing entirely separate directories.
That being said, this is obscure enough that I feel like there should be some kind of check in these clients to make sure they’re not about to interfere with each other - users aren’t gonna know to check for this, especially since these clients are hiding what they’re actually doing behind the scenes!
NUT works with many UPS models and provides monitoring and control
https://networkupstools.org/