I was having the same issue. At some point in the past on the android version of Opera GX, it asked me to "upgrade" to regular Opera. I thought it was just that they were upgrading or merging GX into the main Opera, but that's also the same time this issue started for me.
Going back into Opera GX on my phone, I was able to reconnect my sync between my Android Opera GX and the desktop version without any issue.