The simpler media website CMS
I'm currently running 1.5.7 on a live site without any problem. I also have a development version of the site which I use to test any updates before going live.
I've been trying (intermittently) for weeks to update - first to 1.5.8 then to 1.5.9 but I'm encountering difficulties.
To try to resolve the problems I wiped all files from the development version and reloaded 1.5.7. However when trying to install I get many entries like the following in the Debug log:
NOTICE: Undefined index: CHMOD in /home/bpskdk/public_html/development/zenphoto/zp-core/functions-basic.php(127) : eval()'d code on line 82
eval called from require_once (functions-basic.php )
from require_once (admin-globals.php )
followed by different filenames such as
from admin.php 
from admin-logs.php 
All the errors relate to Undefined index: CHMOD
I have checked file paths and don't see anything wrong but I'm no server expert.
Other attempts to update to 1.5.8 and 1.5.9 on the development site give similar error messages, with the screen often going blank and freezing.
Installation information from my last attempt to go back to 1.5.7 (which appeared to have succeeded apart from the debug log entries above):
Zenphoto version 1.5.7 (Official build)
Server path: /home/xxxxx/public_html/development/zenphoto
WEB path: /zenphoto
PHP version: 7.4.25
MySQLi version: 8.0.27
Any help would be gratefully received.