In Mendix 4.1.0 a configuration item was added that allows you to disable this fingerprinting check:
When running a server using m2ee-tools, you can add this configuration item at the runtime: section of the yaml configuration file.
It could be the user's browser or some plugin in the browser that's modifying the fingerprint. Perhaps the user can use a different browser or reinstall his current one?
This is a safety mechanism so sessions can not easily be taken over, a fingerprint is basically a collection of identifiable aspects of a certain browser. However, it's expected to remain constant for a certain client so if this isn't the case then you will get problems like these.
This will definitely be fixed in 4.3.0 because we are replacing the fingerprint system with something that is a) more secure and b) not dependent on browser (plugin) quirks.
And as Bas says, it probably has to do with the browser on that particular computer. Maybe they have a toolbar installed?
Thanks for the replys. I never had this issue reported when using Mendix < 4. Now just a few days live with a major 4.2.2 application and already five users reported this, the same, issue. Seems to be a serious problem!
Just today the fingerprint error count is 66 in the m2ee logging with just a few users online.
I will file a ticket because this becomes a high prio issue..
Can it have anything to do with using anonymous and users? For example: in one tab visiting a anonymous form an then logging in in another tab?
UPDATE 08-12-2012: I tried disabling anonymous users in a test environment but didn't help. Just had a call with one of the users He had Google Toolbar enabled. After disabling logging in works just fine. Since a lot of people use the Google toolbar you should think that more projects has this issue..