Invalid variable error

Hello,

We are using MII 12.1.9 in a cluster environments.

From time to time, we have some BLS transactions failing on some application servers, with error in NW logs like :

[Transaction: 572463923] Link execution failed: com.sap.lhcommon.exceptions.EvaluationException: Invalid variable: xxx

and

*** MISSING RESOURCE: Unable to localize Unknown variable xxx

So looks like one local or even a transaction variable is not found in the trx, but it's off course well defined!

The only solution is to restart the related servers...

Any idea what can cause this kind of error ?

Thanks