Anyone experiencing this error please let me know. It is affecting only some users because Lpfreaky90 is reporting no issues. Here is a list of threads I found where people are currently experiencing the same issue:

http://www.steamcommunity.com/app/620/discussions/0/522728001004659290
http://www.steamcommunity.com/app/620/discussions/0/522728000979796295
http://www.steamcommunity.com/app/620/discussions/0/522728000875917206

Brumi wrote:
I've also been having this problem for a few days. Has it been fixed for any of you since then?

Unfortunately, I am still encountering this error. However, there is a method that should allow you to play community test chambers even with this annoying error.
1. Unsubscribe to the map that you want to play
2. Launch Portal 2
3. Select "Community Test Chambers"
4. Either choose "Play Single Player Chambers" or "Play Cooperative Chambers".
5. After you choose one of those, you should get a loading screen. After the loading finishes, you should get the extremely annoying error message.
6. After you get the error message, click on "BROWSE THE WORKSHOP".
7. Find the test chamber that you unsubscribed to in the first step, the test chamber you want to play.
8. After you find the test chamber, subscribe to it.
9. The test chamber you just subscribed to should appear in the list of test chambers while the error's text is still behind it.
10. Play the test chamber.
11. If you want to play another community test chamber, you have quit Portal 2 and follow these steps, again.
I know that I added too much details in these steps. However, I thought that I should just in case someone finds a difficulty in following these steps.
I hope this method was helpful, good luck.