Hmmm couple questions.
- When does it start occurring? Like after a certain time on the server, or instantly as you buy a door?
- I've looked around on the web, and someone else had a similar problem. When this invisible door thing occurs can you open sub menus?
- If you exit Gmod, and re launch it do you get an error saying "master pipe lock" or something along that?
- If you restart steam/gmod does it fix it?
Also i'd try suggesting you validate your Gmod files, and see if that work. If even test it in single play wait around for a bit, and see if the doors become invisible. A lot of people are saying on the the web it's a gmod wide issue not really a HL2RP issue. To validate you just right click Garry's Mod on your library > Properties > Local Files > Verify integrity Of Game Cache. I've had this problem before, but I don't remember how to fix it.