Plugin not knowing what admin page it's at

This one looks easy, not for me, though.

I modified a plugin for my client needs, and it works fine in the sandbox wp installation I worked it on, but when installing it in other blogs, randomly it breaks in different ways.

All of them related to one of all the admin pages not containing
/options-general.php?page=
in the name.

1) I found that piece of info in the DB under transient-slug something.
Is that making the plugin to work in the sandbox website?
I haven’t found any info regarding transient slugs.

2) Then, what piece of WP script is in charge of usign/or not that missing part of the URL?

One tip: If I add the missing part of the URL that makes it work in the sandbox installation, it says “I don’t have sufficient permissions”. Different message to what says if I type a non-existing url.

Oh, I forgot: One of the sites just takes me to the blog page (not home).
All of them latest WP versions, default pretty permalinks .htaccess code.

Thanks.