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The current versions of ILIAS usually require the SOAP administration to be enabled for full functionality. Systems that are directly connected to the internet just require to enable SOAP (Administration-General-Webservices) to be enabled.
Some systems located behind a reverse proxy do not work properly with these settings. Some symptoms may be:

• Courses and Categories cannot be copied. The copy process starts, but never gives a result.
• Mails cannot be forwarded to external accounts

 
The ILIAS log shows some messages that the WSDL page cannot be loaded. It seems that the system tries to resolve the DNS name, finds the IP address of the reverse proxy which obviously doesn't supply the WSDL page.
I found two different approaches to solve this situation:

• Additionally to enabling SOAP, enter the WSDL location. Just copy the location mentioned as standard on the webservices page, but replace the system's name by the internal IP address of teh ILIAS server (not the reverse proxy). Save your settings, all functions should work now
• Add an entry to the file /etc/hosts on the ILIAS server with the system's IP address and the system names (hostname and fully qualified DNS name). It is not neccesary to enter al location of the WSDL path in this case. Possibly other plugins and services which try to resolve the system's DNS name profit from this solution too.

It should be possible to point your ILIAS system to an internal DNS server that resolves the internal address of the ILIAS server to its own IP address and to direct clients and other systems to the reverse proxy (so called "split horizon DNS") too, if your DNS infrastructure allows this. I could not run tests for this idea.