Cryptic failed state "Mix engine failed: Array must have size 1, but has size 5"

[WindFreaker]

Manually adding the resource pack Enchantment Outlines in the mixer folder and reloading causes a cryptic warning message to appear.

[05:10:02] [Paper Scheduler Thread - 104/INFO]: [ResourcePackManager] Sending resource pack now.
[05:10:02] [Paper Scheduler Thread - 104/INFO]: [ResourcePackManager] Starting resource pack mixing...
[05:10:04] [Paper Scheduler Thread - 104/WARN]: [ResourcePackManager] Mix engine failed: Array must have size 1, but has size 5

This is all that is printed to logs. No resource pack gets sent to the client.

[R3ner]

Same issue here. Still no solution? :(