Updated to CM 12.1, no recovery mode, everything else seems fine

Today I've updated from CM 11 (KitKit 4.4.4) to CM 12.1 (Lollipop 5.1.1). I made a backup, a factory reset and then flashed the latest nightly and Google apps. Everything was fine so I continued installing apps, restoring data etc. When everything seemed to be finished and the phone was as I want it to be, I wanted to make a new backup.

But now I can't enter recovery mode. When I boot the phone and press the volume button, the LED turns yellow as it's supposed to, but the screen stays black and nothing more happens. If I press the power button the phone reboots. So everything seems fine, the phone is working and I've got root access, but I can't enter recovery mode. I've tried to use ADB and fastboot but I can't connect to my phone so I can't flash a new recovery image. I can boot into fastboot mode with the LED turned blue but ADB stays on <waiting for device>. I've tried to update drivers etc and also tried another computer but with the same result so it seems there's something wrong with the phone.

In developer settings there's an option for CyanogenMod to update the recovery when flashing a new nightly - but I don't know if I can flash a new nightly when I can't enter recovery mode.

Any advices?