Percona Xtrabackup fails to backup InnoDB table with fulltext index
Do I have to do the --export run right after I create the backup ? Or
  can I delay that until I actually need to restore a backup ?


Not necessarily, as long as the backup completes with completed Ok then it is assumed that the backup is successful. It is still best practice to test your backups regularly.


  What can I do about the error for the FTS files ? Is that normal ?
  Will Mysql re-create the fulltext index automatically or is it even
  included in the main tables .idb file ?


This is expected if in your test.txt file the table with FULLTEXT indexes is not included. Basically you are not backing up that particular table so it will be skipped during the backup operation and during the prepare operation

Easily reproduced if you create test1 and test2 tables and then on the test1 table make sure you create a FULLTEXT index. On the backup stage:

181215 10:31:22 [01] Skipping ./test/FTS_000000000000002e_0000000000000033_INDEX_1.ibd.


On the prepare stage:

InnoDB: Failed to find tablespace for table `test`.`FTS_000000000000002e_0000000000000033_INDEX_1` in the cache. Attempting to load the tablespace with space id 30


One more note, it is recommended to use xtrabackup binary instead of innobackupex because starting in version PXB 8.0 innobackupex is removed.