This error can be caused in the following way:
A fixed resource is linked to a state gantt chart that uses uses a state table which was filled based on the state profile of a specific object and does thus not contain all 50 default states.
If the fixed resource is now set to state is not part of its state profile (for example by a time table) the Statistics Collector behind the chart will try to read the display name for that state in the state table but won't find it. It will then try to read the name from the state profile itself, but since the state was not previously in the profile yet and is only being created because the resource is now set to that state this fails as well and results in the error message.
As long as the fixed resource is persistent and does not get regenerated on reset by a parameter setting for example, this error should only appear once, since afterwards the state is part of the profile and its name can be read from there.
In general, if you make sure that the state table contains all states that are going to be used in the model, this issue should not come up.
See the attached model for an example. The first time it's run, you will get error message when the time table sets the processor to 'new' states. In subsequent runs, they will not appear again.
unknown-state-error-fm.fsm
The error could of course be caused by something else in your case. But then it'd be hard to pinpoint the cause without access to the model (or a stripped down version that shows the issue).