Error after uploading picture

[panhead]

After uploading a picture (or more than one) I am often logged out and get this error:

How can I solve that?

[ron4mac]

Ensure that the site URL in your config matches what it actually is (including http:// or https://). This can happen when the site is accessed as https://... and an upload is done, but the config has http://... in the URL value.

[panhead]

The www part in the URL was missing. I'll test it again.