After the release of Web Mail 5.12.0 on December 15, a few users have reported that they can't see the Folders list in the left panel. We've found the cause, and it will be fixed soon (early to mid-February).
In that release, the allowed character list for folder names was expanded to include, among other special characters, the apostrophe. This works fine as long as any new folder you create with an apostrophe in the name is also less than 20 characters in length. But if the name is 20 characters or longer, and contains an apostrophe, your folder tree will disappear as soon as you create the new folder. The reason is that for folder names of 20 characters or more, the display is truncated and we show a tooltip on mouseover that displays the full name. It is actually the tooltip part of the script that is choking, and causes the whole tree display to fail.
If you get in this state, then you can get back to the Folders management view by entering the URL for the folders page in your browser directly (since the link is now hidden from you). Just edit the URL shown in your browser address bar, and replace the part that says "index.jsp" with "folders.jsp" and you will then be back on the folders page, and you can rename or delete the folder causing the trouble.
This bug will be fixed in 5.13, set to release in late February.