To recreate: 
- Set your Default checkout, hold and return policy value for Maximum total holds allowed (count) to 1
- find a patron, confirm you can only place 1 hold for this patron before getting a "too many holds" error
- create a Checkout, hold policy by patron category for your patron's category, with a Total Holds value of Unlimited
- confirm you still get a "too many holds" error, even though your circ rules say your holds should be unlimited

We don't save a max_holds rule with a blank value for the patron category, so Koha ends up falling back to the more restrictive default.