think i've found the problem
Was caused by a too new version of Virtualenv
2020/10/16 12:25:39 Using Python package cache bind mount directory: /opt/rstudio-connect/mnt/python-packages
2020/10/16 12:25:39 Using Python environment cache bind mount directory: /opt/rstudio-connect/mnt/python-environments
2020/10/16 12:25:39 Using git repository cache bind mount directory: /opt/rstudio-connect/mnt/git
2020/10/16 12:25:39 Identified 2 CPUs; configuring Server.CompilationConcurrency: 1
2020/10/16 12:25:39 Configuring licensing to use a local license...
2020/10/16 12:25:39 Detected Python version 3.7.7 at /opt/python/3.7.7/bin/python3.7
2020/10/16 12:25:39 Virtualenv version 20.0.35 in python at /opt/python/3.7.7/bin/python3.7 is newer than the maximum required version 19
2020/10/16 12:25:39 To proceed anyway, set LibraryCheckIsFatal = false under the [Python] heading in the configuration.
i've added the "LibraryCheckIsFatal" line to the configuration file and it works now.
Hope it doesn't cause a problem though!