You CAN send messages to users without OMEMO. Just disable OMEMO encryption in chat with a user who doesn’t have device with OMEMO support. Tap on the lock button in the chat window and switch to unlocked.

Icon in the right of input field is unlocked and inactive, icon in the right of error message is locked and click does not do anything

The issue is then most lightly different. You have enabled OMEMO as default and you have a chat with someone who is not in your roster. To solve that, you can try to change default encryption to none and reopen that chat.

We will try to improve this behavior in future version.

Contacts are in my roster (with "to" subscription)

Ok, switch to "none" encryption solved the problem (I'm not sure I did touch this before)

By default, BeagleIM comes with optional OMEMO support which means that default encryption is set to NONE.