avrdude: stk500_getsync(): not in sync: resp=0x44

A new error... After working fine for days, I now get the error "avrdude: stk500_getsync(): not in sync: resp=0x44" while flashing a sketch to the nano. Serial (via FTDI/USB) still working, flashing via ISP working.

Can someone tell me what the error tells me? Google could not help me and avrdude documentation also doesn't give a hint...