I just moved a gallery to a new server, and the slideshow link on the gallery pages opens a new browser tab all black, no images/slideshow. No errors.

I don't know when the slideshow worked last on the old host, but I think it worked at one time :-)

How can I troubleshoot this problem?

http://robertsivell.com/zengallery

ZP 1.5.6,

Current gallery theme: zpBase

PHP version: 7.4.4
Graphics support: PHP GD library 2.2.5

supporting: gif, jpg, jpeg, png, wbmp, webp
PHP memory limit: 256M (Note: Your server might allocate less!)
MySQLi version: 5.5.5

There is really no error in the debug log? Or the server php error log?

Please try the updated version of the zpbase theme (https://github.com/zenphoto/zpbase/archive/master.zip)
It should work again.

I will try that now...
And it seems to have done the trick. Happy now, don't even need to care what the problem was... totally lacking in curiosity now!
Thank you