can the "LoadProspect=" value be deleted after the first launch of the save or has this to be on? How can I switch between different ongoing prospects on the server? only manually via the ini file?
It can not be deleted; this is how the server knows which save/world to run. Switching between different saves or prospects would have to be done through the config file.
You may have to wait an extra moment in the world before leaving to make sure the proper folders get generated. If it's still not working, please submit a ticket, and we would be more than happy to help.