Backup: Permission denied

I have three webspaces at the same hoster and a local MAMP installation with three small Grav installations.
Locally backup works. I transfered the files with FTP (Transmit).
Website seems to work fine, except backup:
One of the webspaces is new, there backup works.
The other two have existing content (HTML/CSS, no CMS) moved to a subfolder. There backup fails after a few seconds with "Backup: An error occurred. ZipArchive::close(): Can't open file: Permission denied".
Backup size is about 9MB, so the error could be when creating the backup file.
/backup permissions = 755 rwxr-xr-x
I experimented a bit: For Grav installation in a subfolder backup works. Moving all files up and backup stops working.
I cannot see any log of backup describing where it fails.
I do not have CLI, only FTP access.
Resetting file ownership to the FTP user did not help (a tool of my hoster).

Check the backup folder permission for make sure it's 755 and you web host upload size limit. I have mine backup running and working at http://www.drinkharder.com

As I said: /backup permissions = 755 rwxr-xr-x
Why upload?
The webspace is mostly empty <100Mb of 50GB used.

there's a setting in Apache limit size to 2MB. So find out what is your limit.

upload_max_filesize in your php.ini You may want to check post_max_size as well

How could backup run into a upload file size limit?
from grav admin -> configuration -> info:
post_max_size = 200M
upload_max_filesize = 200M

I think I found out: Grav tries to backup every file - even files which are not part of Grav but are created by the webhoster and are not accessible.
=> backup needs a ignore-list

Check this out: https://github.com/leotiger/grav-plugin-backup-manager