OJS e-mail templates: Failed Ajax request or invalid JSON returned error (encoding problem?)

When I open certain e-mailtemplates (for example Copyedit Ack), I get the ajax error: Failed Ajax request or invalid JSON returned.
When I take a look in the php_error.log I see the message: Malformed UTF-8 characters, possibly incorrectly encoded.

Looks like the contents of the e-mailtemplates in the db is in the wrong encoding. Tried to fix this with adding a new db in utf8mb4_general_ci and importing the data again, but this didn’t fix the problem.

Does anyone know a way to fix this?

Does someone have a solution for this?