Unfortunately, WP-SimpleViewer v1.5.4 was retired many years ago (in 2010, I think) when the plugin was rewritten for v2.0.
I realise that this might not be the answer you were looking for but it might actually be easier to recreate your galleries with the current version of the plugin (which is still actively being developed and supported) rather than wrestle with v1.5.4 whose source code neither you not I are particularly familiar with.
Does the error occur in the admin section or when trying to view a SimpleViewer gallery on a web page?
Could you please post the URL of a SimpleViewer gallery page that does not work so that I can at least see the code that is being generated? (None of the galleries on the 'Galleries' tab seem to be SimpleViewer galleries.) It might help. Thank you.
If you'd like me to take a look at your setup, I would be happy to do so but I'd need access to your web server and WordPress site. If you are agreeable to this, then please let me know and I'll send you an email address where you can forward your login details to me.
I can't promise that I'll be able to solve your problem (the best course of action really might be to move on to WP-SimpleViewer v184.108.40.206) but I'm certainly willing to try.
SimpleViewer Support Team