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.

I have the same problem, But i know the fun it is perfectly working because some times it works. The problem is that Acer laptops, are in my opinion not compatible with ubuntu. The fan driver or program that take of the fan it is not working. I tried to download from the acer website but it designed only for windows. I tried to set it up with Wine application, to run windows programs but still need to have the whole operative system.
I have this problem since I installed ubuntu a few days ago.
If anyone knows any other drivers or programs that can manage the fan, please let me know.
Thank you!!