The problem is that this syntax is obsolete since mysql 5.7. From mysql documentation https://dev.mysql.com/doc/refman/5.6/en/alter-table.html:
> As of MySQL 5.6.17, the IGNORE clause is deprecated and its use generates a > warning. IGNORE is removed in MySQL 5.7.

the required console output:
digikam.coredb: Core database: makeUpdates  8  to  9
digikam.dbengine: Failure executing query:
 "" 
Error messages: "QMYSQL: Unable to execute query" "You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'IGNORE TABLE AlbumRoots\n                                        ADD UNIQUE (iden' at line 1" 1064 2 
Bound values:  ()
digikam.dbengine: Error while executing DBAction [ "UpdateSchemaFromV7ToV9" ] Statement [ "ALTER IGNORE TABLE AlbumRoots\n                                        ADD UNIQUE (identifier(127), specificPath(128)),\n                                        ENGINE InnoDB;" ]
digikam.coredb: Core database: schema update to V 9 failed!
digikam.coredb: Core database: cannot process schema initialization
KMemoryInfo: Platform identified :  "LINUX"
KMemoryInfo: TotalRam:  16748412928
digikam.general: Allowing a cache size of 200 MB
QThreadStorage: Thread 0x7ff8652ab100 exited after QThreadStorage 11 destroyed