What should I do if the Production editor has not responded after the proofreading process for over a month? In February 2021, my paper was accepted and the proof reading process started immediately. I have returned the corrections in begin of March, but after that no sign of communication. I send 3 emails to the Production editor, without any luck. How do you recommend to continue?

Finally they replied and told me that they have not received my edits through the online proofing system. I have submitted everything four times and nothing is received by the other end. It is over a month since the proofreading process began and concerns only 9 edits!!! I talked with the eproofing service, but the result was the same. What do you suggest me to do? –
Try a different email. It could be that the production editor resigned, is on leave, has been reassigned, etc. You could try the journal email, if there is one, or the publisher email, or even the editor-in-chief.
Worst case scenario, phone the publisher.