Mystery errors in log, can't track down

[Atreides]

20230212 21:54:47 warning unit: Unknown attribute 'no_recruit' discarded.
20230212 21:58:16 error engine: could not de-serialize unit: 'game_error: creating unit with an empty type field'

The warning I think I read may be a game bug but I am not sure.

The error is coming when I run an era which I've scoured for unit/side tags (0), unstore unit (41), plague (1) and anything else that I read might cause it but to no avail. I'm asking if there is anything else that might be the cause. I've checked all 41 unstore units to make sure they have a valid variable. Plague has a valid type.

I tried a debug log for engine:
20230213 11:57:36 info engine: firing attack event
20230213 11:57:36 debug engine: getting attack statistics
20230213 11:57:36 info engine: Fight: (29,15) vs (28,14) ATT: airgun 2-3(40%) vs DEF: none 0-0(0%)
20230213 11:57:36 debug engine: start of attack loop...
20230213 11:57:37 info engine: defender took 0
20230213 11:57:37 info engine: firing attacker_misses event
20230213 11:57:37 debug engine: start of attack loop...
20230213 11:57:37 info engine: defender took 0
20230213 11:57:37 info engine: firing attacker_misses event
20230213 11:57:37 debug engine: start of attack loop...
20230213 11:57:38 info engine: defender took 2
20230213 11:57:38 info engine: firing attacker_hits event
20230213 11:57:38 info engine: firing attack_end event
20230213 11:57:38 error engine: could not de-serialize unit: 'game_error: creating unit with an empty type field'
20230213 11:57:38 error engine: could not de-serialize unit: 'game_error: creating unit with an empty type field'
20230213 11:57:38 error engine: could not de-serialize unit: 'game_error: creating unit with an empty type field'
20230213 11:57:38 error engine: could not de-serialize unit: 'game_error: creating unit with an empty type field'

It seems to be related to attacks (4 error per attack is the only clue) but even at max. level the log doesn't tell me where.

[gnombat]

I think you might need to provide more details, like which version of Wesnoth you're running and what era this is.

Normally I would suggest following the instructions here but I suspect this issue may involve an old version of Wesnoth so that might not work.

[Atreides]

I did cure the de-cereal-ize error ;-) For reference to anyone searching it can be caused by a unstore inside an (attack type) event that isn't filtered or is nested with first time only no. It was running all the time when it was part of a special...