The path to the project´s .xml file when running ReadyAPI tests is fixed.
When several users are using this project they all have to install the project in the same file-location since relative path is not supported by TC.
Response from SmartBear when addressing this issue:
Unfortunately, it doesn't look like TestComplete supports relative paths for this field. ${projectDir} is only available in SoapUI and the documentation is only relevant for SoapUI. I have spoken with a few team members and there does not seem to be any alternative to hard-coding the path.