Can you tell us how you paid your bills today and recently? Browser? Were you in bill lock or just doing your normal thing?
I've had issues in the past but never tried to replicate them since, where using a mobile device I clicked to pay bills and in fact it never did, later did I realize when using a computer that my bills had stacked up for several days.
If you really did just get that error for no reason see if you can declare war on someone in your range without actually hitting declare war. Also check if you can purchase military items. If your bills have truly not been paid then those things would also be locked meanwhile if you can access buying things like tanks, nukes, aircraft, etc, then this is just a glitch with your aid system. (Presuming the person you're sending to also has the slot open to receive the aid)