If I had your (sambamm2010) issue, I would reinstall and login to same dropbox account. After that go into your android gallery look for elipse (3dots) and choose settings -cloud sync and disconnect/remove the dropbox connection. After that reboot the phone. At this point if the problem persists, uninstall dropbox and reboot the phone. This should clear the issue. I think your dropbox connect to android gallery is corrupted and why you cant access the gallery now.
The key to the resolution is that you should disconnect the android gallery from dropbox sync FIRST before you remove dropbox. Why? Dropbox app cant make a call to the gallery settings to remove sync connection during uninstall process.
I would leave Pokemon Go off because it is not secure and can give location data.- says a non-gamer
GL