I suspect these several issues, three so far, are all related:
- this one you just posted above
- nbgitpuller ceasing to work now, see the Discourse thread here and the related open Github issue here.
- fresh build of images for my Binder using the r-conda repo as a template not working to allow switching to RStudio, either using the URL or the RStudio tile in the launcher, see the related Discourse thread here.
They all involve MyBinder sessions parsing the URL and doing something beyond the basics of switching between JupyterLab and classic interface. I just posted about my suspicions about two if these here.