From my point of view this is purely an issue of the GCM app with certain phones / Android versions. With a super clean Android directly from Google but on Marshmallow 6.0.1 I was no longer able to update the EPO.
Chaning to a newer phone with at least Android Oreo (v8) it worked again without any further issues.
I strongly suggest to look here if you want to analyze any issues with that.