I finally have figured it out and solved it.
I changed the “From Email” address in each form to a different one than the one to which the notifications are sent. And it works.
I guess my provider (Protonmail) was rejecting those notifications because it appeared that I was sending myself messages using another provider, which is impossible. They probably rejected those as fake/spoofed emails.
In case anyone else encounter this error, try this: WordPress > Forminator > Forms > Edit Form > Email Notifications > Edit > Advanced tab > From Email (enter an address different than the one receiving the notifications which is written in the Recipients tab).