Updating plugins no more possible

In WordPress I no more can update plugins. The message is ‘Actualisation failed: -1’.

I read in several websites that this has something to do with the file ‘. maintenance’, that is not deleted automatically by WordPress. So I delete it in FileZilla. But the message keeps popping up after again trying to update a plugin. I followed all hints, for instance that the plugin should be compatible with WordPress (which turns to be OK).

What next?

Thanks.