I’d certainly try deleting the app from your iPhone, waiting a couple of minutes, then download a fresh copy from the iOS store. Make sure you ‘join existing system’ when prompted, and see if it continues to ‘flash’. If it does, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Has been happening every few days for me since the last update.  Try reinstalling and see if it fixes it, at least for a day or two. Then repeat. 
 

Thanks guys, I’ll give it a go

deleted and downloaded the app….  all signed up and working.

Thanks again guys