Acrobat 5.2 adds full support for Protected Mode in Acrobat in Windows. If you try to use older version of Quite Imposing Plus, you will have major errors such as being unable to save license information, unable to remember any settings, and unable to use automation sequences.
Since 2020, Acrobat has offered the option of “Protected mode” in Windows. They are now using it as the default for new installs, so this will eventually affect most Windows users. This provides protection against unknown threats to your computer. Because it stops Acrobat from opening many files, it can cause plug-ins to fail, including Quite Imposing Plus. Version 5.2 includes a “plug-in broker” which allows Quite Imposing to ask you for access, while still keeping full security.
This is installed with Quite Imposing Plus, and automatically used if you switch on Protected Mode. Protected mode is controlled from Acrobat’s Preferences under Security (Enhanced). Look in the “Sandbox Protections” section for the option “Enable Protected Mode at startup”.
When you first open a file, Protected Mode is not involved. It becomes an issue when the file needs to be opened again in another Acrobat session. For example, if you want to use the same page background or data source as before. You may see a prompt like this one:

So long as you are expecting the access, you can allow it. You can allow access to the file, or to all files in a folder. An allowed access is remembered until you quit Acrobat.
Access to a folder does not apply to subfolders inside that folder, which must be allowed separately. We do not recommend you allow access to folders that may have other kinds of file, such as Desktop or Documents; this may be blocked.
For several years, Acrobat has offered the option of “Protected Mode” in Mac. Adobe are now using it as the default for new installs, and it may soon be impossible to turn it off. Quite Imposing Plus 6.1 adds full support for Protected Mode in Acrobat in Mac.
If you try to use older version of Quite Imposing Plus with Protected Mode, it may refuse to run, or you may (in older versions) have major errors such as being unable to save license information, unable to remember any settings, and unable to use automation sequences.
Protected Mode provides protection against unknown threats to your computer. Because it stops Acrobat from opening many files, it can cause plug-ins to fail, including Quite Imposing Plus. Version 6.1 includes new features which allows Quite Imposing to ask you for access, while still keeping full security. In Mac jargon, Protected Mode is called a “sandbox” and many Mac apps are now required by Apple to use a sandbox.
This is installed with Quite Imposing Plus, and automatically used if Protected Mode is switched on. Protected Mode is controlled from Acrobat’s Preferences under Security (Enhanced). Look in the “Sandbox Protections” section for the option “Enable Protected Mode at startup”. If you do not see this, Adobe have presumably removed the option. Note that “Enhanced protection” and “Protected view” (if they appear) are entirely different things.
When you first run in Protected Mode, Quite Imposing Plus will set up so that you can read and write settings files, licensing information and the other things needed to run. You’ll see this message

You will normally choose Setup now. If you choose Don’t ask again today you can continue in Acrobat without Quite Imposing Plus. If you want to set up at any time, look in the menus for Plug-ins > Quite Imposing Plus > Setup now. After this you will need to restart Acrobat.
If you
choose Setup now you will see a prompt very similar to an Open dialog. 
You need to choose Allow access. Don’t click on any different file or folder, you have to allow access to that specific Quite folder. (Prompts may vary).
It may be necessary for the plugin to run setup again, for example after some kinds of migration, after reinstalling Acrobat, and other situations. You will see the same prompts, and once you set up you can continue as before.
When you first open a file, Protected Mode is not involved. It becomes an issue when the file needs to be opened again in another Acrobat session. For example, if you want to use the same page background or data source as before. You may see a prompt like this one, very similar to the setup prompt:

So long as you are expecting the access, you can allow it. Allow access automatically unlocks access to all files in that folder, until Acrobat is restarted. Note that Acrobat will unlock access for ALL plug-ins together, not just Quite Imposing Plus, and currently it also includes all subfolders. (Acrobat or the system might change this).
Sometimes the Mac system will add extra prompts to access files; these are not under our control.
It can be difficult to understand Protected Mode and the implications of turning it on and off. This is an Adobe feature, and Adobe may change it without notice, but here is an attempt to explain it.
In Protected Mode, Acrobat only allows access to files that you have chosen to use in the current session, for example, files you chose to open. This is a big problem for plug-ins like Quite Imposing Plus, which may want to use files you chose before (such as a PDF used as a background, or a CSV data source). So, from Quite Imposing Plus 5.2 (Windows), 6.1 (Mac) you can have the benefit of Protected Mode and still use these files, by confirming access to them.