error/moodle/dmlwriteexception
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.
MySQL
If you're using a MySQL database for your Moodle installation, this error can be caused by the server's max_allowed_packet size being configured incorrectly. Increasing this value may resolve the issue.
dmlwriteexception error when restoring a course
If you obtain a dmlwriteexception error when restoring a course, it is recommended that InnoDB tables are converted to the Barracuda file format. See the section 'Converting InnoDB tables to Barracuda' in Administration via command line for details of why this is recommended plus information on a tool for converting tables.
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.
