Specifying a BoilerPrototype::target_temperature which is outside of the allowed temperature range of the output fluid will cause a prototype error.
Changes in version 2.1.21 make the maximum temperature of steam 4015 degrees, and if it exceeds this, an error will occur.

ty for the report, will check out

i will move the steam buff to data-updates stage, maybe some mod is nerfing it. RN its set to 10k in data stage

Error ModManager.cpp:1782: Error while running setup for entity prototype "tur_exchanger_entity_3" (boiler): target_temperature (10000) must be within the temperature limits of the target fluid (steam).

In version 1.2.24 the error still happens, but setting the temperature to 4015 fixes the problem.

thats so weird, try just UTF alone please, its some mod changing it. also, send your factorio current-log

i can add a setting to force it in data-final-fixes btw