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What probably could also help: If there is a key on the foreign table affecting the field to be used as foreign key: recreate that index. (Not tested)

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Solutions for case 2:

Solution A - inconsistent data types / collations / encodings

See https://stackoverflow.com/questions/16969060/mysql-error-1215-cannot-add-foreign-key-constraint

One example to get the error: 

1. Livesystem DB System or DB itself is using encoding and/or collation A as default
2. Testsystem DB System or DB itself is using encoding and/or collation B as default
3. User makes a dump from live and applies it to Testsystem
4. User then tries to update the Testsystem
   1. The dump from live creates table LEK_task with encoding/collation A
   2. The Update tries to create table XYZ with collation B
5. => Result: the foreign key could not be created

Solution B - innoDB foreign keys corrupt

In that case the error happened on Updating data in the LEK_term_attributes table. Error was:

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