Update to Bug 1275. It seems the "switch user" functionality in RHEL7 requires display 1 in order to work, thus fails if a TL session with display 1 is running. We could revert to 100 since 1.4 clients are no longer relevant. Or, we could consider using 10. This is what sshd uses by default.
Since both ThinLinc and SSH does check if DISPLAY number is allocated, we can also go with 10 as a starting number.
Seems to work correctly. A default installation gets the first session at display 10, the next at 11, etc. Changing the configuration back to 1 gives the old behaviour. Also couldn't find any place in the source tree that was overlooked.