Seems you are not alone with this problem reading this thread below.
Perhaps after you have updated the box, go into the box settings under System > About and post back the software version number.
I don't know what's the latest version on the older black boxes, but my NowTV 4K box is running 9.0.3 build 4114-68.
All i can do is to tag the NowTV Community Forum Manager @Karl-F and when he his hopefully back in the office on Monday or early part of next week he can then pass this information onto his Technical Dept to investigate this further and hopefully get back to you with a fix.
I have just dusted the cobwebs of my old nowtv black box and checked the latest software version on the box (after i forced an update on it) which like you is currently running 9.0.3 build 4113.
Played a couple of YouTube videos which seemed to play fine, then navigated around the NowTV App for about a minute (box not currently registered as one on my six devices), then when i pressed the Home button on the remote the screen locked up on me and i am stuck on a blue screen with the NowTV pink logo top left hand corner, roku powered and options displayed on the top right of the screen and nothing else showing.
So it appears to me there is something not quite right with the older nowtv black box and the current software from my quick test.
I could reboot the box by temporary disconnecting the power, but if this happens regularly then it needs investigating by the NowTV Tech Team.
I posted on the other thread, still having problems with all 3 boxes, all freezing then stuck on start up screen. Have factory reset all boxes and still the same issues. I have to unplug for a few minutes and plug back in and then they work again. Even rebooted my modem and settings. Software running is
Software version: 9.0.3 Build: 4113 on all 3 boxes.
Seems like a couple of Roku 3 owners which the older nowtv black box hardware is based on are encountering the same exact issue, where their devices are in a continuous boot up loop.