Hi,
Just to add to this discussion, we have 2 mirrored installations of a site on 2 different 1and1 hosting accounts (Europe). One of them works fine with CF7 version 4.6 but the other throws the same error as outlined above.
Both sites are on WP version 4.6.2.
We tried both the rollback to 4.5.1 and editing line 18 of includes/functions.php in the 4.6 version of the CF7 plugin’ from wp_get_upload_dir() to wp_upload_dir(). Both these options worked, so I don’t know what it is between our 2 different 1and1 accounts, but thought I would share this info for anyone having problems.