mark as solved pls, you can unmark it when the problem reoccurs. BTW, just a Q, with the case open, do you hear the fan turning? Did you check if the fan silent mode in BIOS is set to on, Did you check the fan RPM in BIOS to see that it is at 0Rpm's before you opened the case?

I can't seem to find anything about the fan in the BIOS settings and yes with the case open I can hear the fan better.

Since yesterday I have been using the laptop with the case closed (for at least a few hours) and haven't had any problems.

Try this, on my PC the values are posted in the Bios under PC Health Status, there you can see the CPU temp and CPU fan speed aswel as system fan speeds if you have a compatible system fan wich should variate between 1500Rpm to 6000Rpm dependend on CPU temp.