Hi Michael,
There was a header as well, but I changed it to just the [newsletter] shortcode and nothing more, and it did not help. Besides, the newsletter was working well before—we had it on our page for several months.
I have just tried to subscribe again and found out that it redirects to this link: wilda.eco/en/en/newsletter/?nm=confirmation&nk=113-923e0519de
When I remove the double /en/en/ from the link and open it, it shows the correct page. The same is true for the confirmation link.
It does not work however for the Polish version, although the /en/en/ is still added there. The link looks like this for the Polish version:L https://wilda.eco/pl/en/en/newsletter/%3fnm%3derror#TRPLINKPROCESSED
Our page does not have the /en/ for the default English language. We use Translatepress.
Once again, it had all worked well before for both languages.
Best regards,
Adam