CF7 and Flamingo randomly malfunctioning

We are having a weird issue: from time to time (I’d say 5 or 10% of times, so not most of the times, but still important) CF7 form doesn’t work. It’s not that it shows as sent, it just stalls and does nothing. This started two weeks ago and it seems to be more frequent every day (from 1 failed mail every 2 or 3 days to 1 or 2 mails per day, at least that I know of because teh sendders contacted us from our sources, could be more but they didn’t contact us)

I activated debug log and ran many tests and couldn’t replicate it. However, I see in the logs this message, both for CF7 as well as for Flamingo (hence why I mentioned it)

PHP Notice: Function wpcf7_special_mail_tag() was called incorrectly. The fourth parameter ($mail_tag) must be an instance of the WPCF7_MailTag class. 

then some more text that tells me which plugin is (CF7 or Flamingo), but basically the same message on all test messages I sent. While all those messages actually worked, I’m wondering if this could be a reason or at least give some kind of hint on what could be happening.

Just in case, we’re using Contactform7 Redirect Settings and Google Sheets Pro but as far as I can see there are no errors with that, and if the form is sent, both the redirect and the Google Sheets connection work.