UPDATE - FIXED! See release notes.
Since 5.11.2 was released we've had a few reports of users who are using the preview mode (Show Preview at upper right of page) and they report not being able to click and select messages to view. The first message in the list shows up fine, but others can't be selected.
You can turn off the preview mode until we get this fixed, and then messages will view fine. This can be toggled at the upper right of the page by clicking Hide Preview, and can also be set in preferences.
Check this other bug report for some other possible workarounds.
If anyone can provide more information to help us reproduce this, please post it here.