Please read release notes:
https://github.com/wekan/wekan/blob/master/CHANGELOG.md#v300-2019-07-25-wekan-release
Master branch now uses MongoDB 4.0.10, so it requires that you backup your old database https://github.com/wekan/wekan/wiki/Backup with mongodump and restore with --noIndesRestore included. Master branch has newest Meteor 1.8.1 and Wekan newest version v3.00, it requires new MongoDB version newest 4.0.10 or at least 3.6.x. Devel branch has old Meteor 1.6.x version that works with MongoDB 3.2.x and not newer MongoDB.