Those are previous messages from the upgrader:
2018-08-29 10:58:49.991063+0200 Lengo[3143:1631360] CouchbaseLite Info: Upgrader upgrading db </var/mobile/Containers/Data/Application/8795402F-5CCB-4DE5-8D6B-B9CA098B68D0/Library/Application Support/CouchbaseLite/app-data.cblite2/>; creating new db at </var/mobile/Containers/Data/Application/8795402F-5CCB-4DE5-8D6B-B9CA098B68D0/Library/Application Support/CouchbaseLite/app-data.cblite2_TEMP/>
2018-08-29 10:58:50.019607+0200 Lengo[3143:1631360] CouchbaseLite Info: Upgrading CBL 1.x database </var/mobile/Containers/Data/Application/8795402F-5CCB-4DE5-8D6B-B9CA098B68D0/Library/Application Support/CouchbaseLite/app-data.cblite2/>, user_version=102)
then: Importing doc “My Docs” multiple times and after that the error as above.
For my tests I am using the latest iOS Version 11.4.1.
In case you have any idea what I could try to make it work, please let me know.