Juraj, investigating your first hint with the domain name, I ended up adjusting on the target site the Sites > (site name) > Edit > Site domain name:* to
teacherportal-stg.rXXXc.com:443, initially there was only
rXXXc.com. That solved the workflow import problem.
However, another issue is that the imported workflow is missing the whole Scopes section (Workflows > (workflow name) > Edit). I did check the
Import all workflow scopes on All Objects > Global objects > Configuration > Workflows. Any ideas what am I missing?
When I did the same import procedure to our QA server, the Scopes are visible fine.