Input field für html dissapeared - No campaign possilble

After upgrading to v.3.6.2 there is no html input-field at the first page of a new campaign. I can see the headline (Nachricht erstellen) but no field to fill in.
For me that would not be a problem, because I only send text-mails. But if the html-field is empty (or missing), I get an error-message, when starting the campaign and nothing works.

I tried to reinstall the files manually, but have the same problem afterwards.

Is there anybody, who knows what’s wrong?

@prediger Can you add a screenshot of the compose page?

Thank you @duncanc for responding. Originally I didn’t find hwo to upload files. Now I’m better…

@prediger Do you have the ckeditor plugin enabled? If so try disabling it, or enabling it instead.
Also, can you use the browser tools to see whether there are any javascript errors?

Thank you @duncanc. This did the job. I had the fckeditorplugin enabled. Now I disabled this and enabled ckeditor and everything is fine.