Hi @almolo.
The Cobertura format is currently not supported for .NET in SonarCloud/SonarQube (which, unfortunately, isn’t obvious from the error message).
If you want to keep Cobertura, then you can use multiple formats, e.g. binary and Cobertura. Then use the one in binary format for SonarCloud and use the one in Cobertura format for other tasks.
See a couple of examples here. Is any of them useful for your issue?