Because we are using liquibase as DB versioning tool, i took the postgresql upgrade script from camunda-sql-scripts-7.7.0.jar and created a changeset. After deploying everything was fine, but when using delete scripts from the engine 7.7 jar tables were not dropped because of the following missing index in the upgrade script:
create index ACT_IDX_EXT_TASK_ERR_DETAILS ON ACT_RU_EXT_TASK(ERROR_DETAILS_ID_);
After adding it to my liquibase-script, everything worked as designed.
This is only to infrom you.