Reader properly parsed config (and that was OK), but DefaultTypesConverter decided to accept anything for fileds of type String, BareJID, JID as it used String.valueOf() on the value and the result converted to String, BareJID, JID.

To fix that, I've added code checking if the field of type String is not accepting Collection or Map. That fixes this issue and is a valid generic check for those fields.

[2021-03-03 19:28:40:343] [WARNING ] [                  main ] AbstractBeanConfigurator.configure(): Can''t prepare value of property 'mainVHostName' of bean 'instance': '[test-domain.com]'
java.lang.RuntimeException: Cannot convert [test-domain.com] to class java.lang.String
	at tigase.kernel.DefaultTypesConverter.convert(DefaultTypesConverter.java:160)

Works as expected.