@Tuny my knowledge about serial communication is rather limited thus I have not much to add here. Having a quick look at the code of the smartmeter binding it just uses the available core libraries for the serial connector, just as the comfoair binding does.
From a rough read over this thread I’d assume that your connection to the smartmeter is just more unstable than that to the comfoair, e.g. due to interference, bad/longer cable, etc., which then causes the connection to get blocked.
However, I just saw, that the related bug in the error handling of nrjavaserial is fixed upstream and should find its way in OH soon (see Serial ports getting blocked after some re-connecting · Issue #1842 · openhab/openhab-core · GitHub)
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