Did you migrated to SR4?
Whe have the same issue after migrating to QV10 SR4 with customers that only have Named User CAL's and Document CAL's and, as you described, it's a random issue. Some users can access the document, some user can't.
There is a bug in SR4 related to the error message you described.
1. I think you have to open a ticket with with QlikTech Support (or QlikView reseller you are working with) in order to get Customer Patch download links for this bug.
2. After patch installation, you may need to delete .pgo files and reassign licenses. I't seems like someting get broken at .pgo files.
Note that you can find .pgo files into two folders:
1. Root folder as configured into QEMC.
You have to delete .pgo files at both folders.
I suggest you get back up of .pgo files, so you can restore them if something goes wrong.