Code mode e-mail - e-mail builder not accessible

My Mautic version is: 4.4.3
My PHP version is: 7.4
My Database type and version is: mysql

My problem is:
When creating an e-mail and selecting “code mode” as a theme the builder button on the right upper corner is greyed out, the builder itself not being accessible. GrapesJS is installed and running an I have also cleared the cache. So we cannot use the builder to adapt e-mails we made previously nor create new ones with “code mode”.

These errors are showing in the log: none

Steps I have tried to fix the problem: check GrapesJS and clear cache

As far as I could see there is no solution to this problem on the forum yet. Could anybody lend a hand / advise us what to do?

Many thanks in advance,
Bart