Watchdog file growing out of control
You have a few options here.

For the short term, you can disable database logging on /admin/modules/uninstall until you can get the full situation under control.

You can disable assertions.  Search yout settings file(s) and set

assert_options(ASSERT_ACTIVE, FALSE);


You should also edit your php.ini file to set zend.assertions=0.

You can then enable database logging again.

That said, it seems like you are running an old version of Drupal.  PHP 7 changed the way assertions work, this was fixed in core in late 2017 and was part of Drupal 8.5.  You should update your site to fix this, and get the security updates that have been made since then.