I had several errors with empty STACK message but never this short.
Edit: looking at my current error message the F\Brand means that it is connected to field Brand. Try checking if the field exists, where you use it and if it is connected properly. If is is used in filter then check if it is set to existing values.
Generally I would start with reloading metadata and trying again.
If it does not help locate the cause by removing objects one by one until the report works again. (Do not save and use only previews so that you do not ruin your template.)
Also maybe in preview you will get more detailed error message.