I'm trying to search for Common/Uncommon, Non-Attunement weapons (Light Hammer, Mace) with an Effect Type/Subtype of Damage/Fire and a Rating Threshold of Zero.  It manages just fine to search with Rarity: Common+Uncommon / Attunement: No / Rating Threshold: 0. However, as soon as I try and add a Weapon Type, it takes forever to think and just returns an internal server 500 error.

What is Rating Threshold? I don't see that filter in the search...

They're searching homebrew magic items.

Sorry about that, I just edited the title to say Homebrew Magic items

 I replicated the problem exactly as the OP described. In my Firefox developer tools, it found the following errors:

Uncaught TypeError: properties is undefined

    pushToDataLayer https://www.dndbeyond.com/homebrew/magic-items?filter-type=0&filter-type=9&filter-search=&filter-rarity=1&filter-rarity=2&filter-requires-attunement=f&filter-effect-type=2&filter-effect-subtype=51&filter-weapon-subtypes=10&filter-weapon-subtypes=11&filter-has-charges=&filter-author=&filter-author-previous=&filter-author-symbol=&filter-rating=0:28

    <anonymous> https://www.dndbeyond.com/homebrew/magic-items?filter-type=0&amp;filter-type=9&amp;filter-search=&amp;filter-rarity=1&amp;filter-rarity=2&amp;filter-requires-attunement=f&amp;filter-effect-type=2&amp;filter-effect-subtype=51&amp;filter-weapon-subtypes=10&amp;filter-weapon-subtypes=11&amp;filter-has-charges=&amp;filter-author=&amp;filter-author-previous=&amp;filter-author-symbol=&amp;filter-rating=0:303

magic-items:28:13

and also:

Uncaught SyntaxError: missing ] after element list magic-items:2211:43
note: [ opened at line 2210, column 42 magic-items:2210:42

Edit: searching for just "Weapon Types: light hammer" is fine; but "Weapon Types: mace" gave the error. Possibly because there aren't any published homebrew maces that match those filters?

Edit2: No, there are maces that match the filters. I found that searching on "Rarity: common, uncommon", "Weapon Type: mace" is the lowest sufficient to cause this error.