One reason for the error is that your database encoding is not properly configured.
Please check your database encoding and collation.
SELECT @@character_set_database, @@collation_database;
SELECT character_set_name FROM information_schema.columns WHERE table_schema='omp_master ' AND table_name='email_templates_default_data' AND column_name='subject';
SELECT default_character_set_name FROM information_schema.SCHEMATA WHERE schema_name = "omp_master";
If the answer is not utf-8 , you can run a conversion of the database and the tables using the following script.
PLEASE BACKUP YOUR DATABASE BEFORE RUNNING THIS SCRIPT, CAUSE WE CANNOT TAKE ANY RESPONSIBILTY FOR DATA LOSS.