Filter by participants function for conversations doesn't work

Steps to reproduce:

1. Open conversations page https://swat-portal.com/forum/wcf/conversation-list/

2. On the right, start typing a name of someone you had conversations with in the past, and select the name from the menu that appears.

3. Press Submit button.

The page will try to load for a minute or two, and then the following message is displayed:

"The page you are looking for is temporarily unavailable.

Please try again later."

Next line below the message contains a long sequence of hex numbers.

I tried 3 different user names and got this result every time.