I'm happy to post that I found a solution to this problem. I had the same problem on my 1998 Boxster. The first problem I found was the linkage to the NSS. I replaced it and soon realized this was not the only problem.
The next problem was the Shift piston in the Valve body. Don’t worry it was easy to replace and it cost $23.00 Shipped. I have attached a link to an article that explains it all. It says 1997 only but mine is a 1998, it may be an early 1998 so that could be the reason I had the same problem.
https://www.transmissiondigest.com/1997-zf-manual-valves/
I hope this helps you!