i had the same thing happen to me, you just have to slightly tap it. At least that's what worked for me, but indeed the issue is annoying.
The only way I was able to fix this, was archiving Minecraft and then reinstalling it. But it's very troublesome and time consuming to do if you want to play different maps with your friends. (I play on switch)
I have a similar issue, but instead of the export being a success, it says "we couldn't pack your world file for upload. Maybe your hard drive is full?" But it's not. I have 15GB of space left on my console and an entire 8GB SD card. Please fix this issue asap. Thanks.
I'm experiencing the same issue on Windows 10 edition.