This indicates that a general error occurred when Moodle tried to write to the database. If you turn on Debugging you will get more detailed information about what the problem is.
When importing questions in Aiken format after successful reading all questions this error emerges when saving in the database. No definite rule why. Ver. 2.0
dmlwriteexception error when using calculated questions in a quiz
If you're using MySQL or SQL*Server and you have a problem with duplicated keys with the question_attempt_step_data table when using calculated questions in a quiz (from entering a formula which uses variables with the same characters in different cases), it is recommended that you upgrade to Moodle 3.0.x or higher ASAP. Alternatively, the problematic unique index can be dropped or the collation of the columns changed to be case-sensitive, however this is not considered a complete fix. See MDL-29332 for more information.