It’s very unusual for a blank page to not be accompanied by a server error message (generally Error type ‘500’). Note, I do mean server error log, not kiwitrees logs in admin.
Is this a new problem? It’s not been reported anywhere else, and there were no changes to the messaging system at the last update, nor are there any PHP version issues related to that code. Those points, and it happening today on both your sites definitely suggest a server issue.
Have you tried:
Waiting to see if it clears, as noted earlier
Unless one of those suggestions is of any help, there’s nothing more I can offer. It’s not a bug in kiwitrees that I can identify.