Bookmarks will be stored within the shared-files and they could be corrupted. Within the power-tools are a viewer- and repair-tool for them. Beside them it will be useful to backup these files regulary.
When the bookmark is created/saved, Ask the users not to check the layout State. This will reduce your bookmark corruption.
Saving Bookmark from the Browser, using Ajax Client has a limitation on the size of the bookmark - 32k and this layout will hold more than this size which will cause corruption.
If you want a better user experience - Use PowerTools (as suggested by Sommer), to identify the corrupted shared files and repair it. Backup the shared files at regular intervals.
if you want to find which shared file to repair - watch for "Error Shared - FileRead" in event logs for the respective Dashboard which needs a cleanup.
So after some time passing I managed to capture a user that it happened to and I raised a case with Qlik to get it resolved. Their first thing that they suggested was to check the server version.
We recently upgraded the servers and we have 1 backend and 2 clustered. It turned out that the cluster had not been updated to the same version. I then found out the very latest update to SR15 handles some stability issues to do with bookmarks. So I updated the environment again and it now seems to work.
I would try doing thisellyodelly