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.
dmlwriteexception error when trying to save page that has content in texteditor
The most common reason for Moodle users to having this error is when copy-pasting materials from outern source e.g. MS Word or webpages. If the source material contains formattings the source code may not entirely get copied and when pasting the material the source code has open tags or other formatting. The result is broken code that leads to dmlwriteexception. You can paste the material with paste special option or clear formatting (or simply just copy-paste the text to text-editor and then copy-paste the text to Moodle).
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.
Another reason on MySQL can be that the
max_binlog_cache_size is too small. This can be increased up to 4GB max on MySQL 8.
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.
dmlwriteexeption error when importing quiz questions
Make sure the uploaded file is UTF-8 without BOM