Same here, for the Ultimaker S3: now and then a black screen for which only a power cycle reset remedies it. This morning I was expecting to see a full 5-day print finished, to arrive at a paused (PVA flow error) build with a black screen. I was relieved to be able to resume the print from Cura monitor interface, but I'm convinced this merits attention from the firmware devs. Attached the zipped logs I could extract from http://{printer_ip}/info/log.html
log.zip