Common-Issues.md

+## General
+Usually trying the following resolves all issues:
+* Invalidate cache and restart (JetBrains IDEs).
+* Make sure `application.yaml` contains the latest changes made to `application.template.yaml`.
+
+## Mail Server Error
+##### Where?
+Usually in Queue, during the first start-up.
+##### Solution:
+There is a setting in `application.yaml` called `test-connection` in the `mail` section. Make sure this is set to `false`.
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