I have this issue:
flashed 26.04 nightly and my phone start rebooting constantly (like a soft reboot). Then tried to flash last known working nightly 20.04, but gets fc (processes: media, phone, messaging, email & so on). Last backup is from January and don't want to restore it yet, because i'll loose too much data
What CM version is the 20th nightie - 12 or 12.1? If 12, you might want to flash also the appropriate GAPPS. That changes with Android 5.1, a.k.a. CM12.1.
Still it's strange 26th nightly works like a charm for me.