The only weird situation i remeber with particular or complete frozen target visu was after the rtc clock was fallen or set back to the past. It looked like after running a while the alarm history database crashed during writing entries at a date lot of years in the past. After setting the rtc clock to actual date/time and deleting the alarm db every thing worked fine. I don't know, if this is/was a problem of our PLC hardware/firmware? Maybe check date/time at your system?