Good day   @langlois,

If you check at our compatibility matrix for the supported versions of the .Net version you will notice that .Net framework 4.8 is not listed hence you are receiving the error.  

I believe you are right if you would uninstall the current framework that is installed and consider installing the frameworks that have been specified for K2 five in the product and compatibility matrix page: https://help.k2.com/k2compatibilitysupportmatrix

Good day  @langlois;

As mentioned by  @Dumisani  ,please note "It is always recommended that the latest versions of K2 software be used as newer versions may contain performance improvements. This can be done by reviewing the release notes of latest K2 software versions that are released."

Hope this k2 5.3 release notes will assist you(https://help.k2.com/kb002966).

Best Regards;

Widson.