What happened? 

The Prusa CORE One, and the Original Prusa XL with Enclosure, have chamber temperature measurement. 

In certain conditions, the chamber temperature is measured as critical. In this case, the printer stops the print and shows a warning message "Chamber temperature critical. Aborting print to prevent printer damage". 

Error name: Chamber critical temp

Error code: #31261 (CORE One) #17261 (XL)

How to fix it? 

On Prusa CORE One, cool down the chamber by setting the chamber fans speed to the maximum, from the printer LCD Menu -> Control -> Temperature -> Chamber Fans.

At the same time, open the top lid ventilation holes, by sliding any of the three indicated screws. 

On Original Prusa XL with Enclosure, set the enclosure fan speed to the maximum, from the printer LCD Menu -> Control -> Enclosure settings -> Manual settings -> Fan rotation. 

At the same time, slide down the bellows.