toucanbuzz
No rule is inviolate
Per Sage Advice official rules clarification, if you don't have the War Caster Feat, you have to "put away" your weapon or shield in order to cast a spell with a Somatic or Material component. Per clarification on what it means to prop or put away an item, Sage Advice clarified you can hold an item in another hand to free up a casting hand (e.g. hold a quarterstaff with one hand).
So, like others have said, I think it'd be fair and consistent to allow the Knight to briefly prop his mace and shield into one hand while he casts the spell. In my games, I rule he loses the benefit of the mace and shield until his next turn because it's pretty hard to use either while propping. If you want to use one and still cast, you'll need to drop it (free action, then an action to retrieve an item). This gives the War Caster Feat actual meaning. Otherwise, it would create an illogical result (why make a feat if everyone already gets the benefit of being able to cast with full hands and no penalty?)
Finally, this differs from the Cleric, which specifically is allowed to have the shield function as a holy symbol for the spell material component. However, if there's still a Somatic component requiring hands, the rules apply as regular.
So, like others have said, I think it'd be fair and consistent to allow the Knight to briefly prop his mace and shield into one hand while he casts the spell. In my games, I rule he loses the benefit of the mace and shield until his next turn because it's pretty hard to use either while propping. If you want to use one and still cast, you'll need to drop it (free action, then an action to retrieve an item). This gives the War Caster Feat actual meaning. Otherwise, it would create an illogical result (why make a feat if everyone already gets the benefit of being able to cast with full hands and no penalty?)
Finally, this differs from the Cleric, which specifically is allowed to have the shield function as a holy symbol for the spell material component. However, if there's still a Somatic component requiring hands, the rules apply as regular.