@RawConvert Actually, my two computers use UEFI and were up to date when I built them. The motherboard manufactures website showed compatibility with my CPU and ram. Considering how stable these two machines have been, I have no reason to believe that the UEFI is at fault.
I'm thinking the problem may be with GTK3. I have used many versions of rawtherapee-unstable from Dariusz Dumas PPA and have not had a failure. The unstable packages appear to be built on the Master branch which appears to use GTK2.
Since you are using a newer release of Ubuntu, your shared libraries may be updated. This could be another reason.