Hi Sefton,
Sorry but to start with Acro-9 is no longer supported by Adobe. That means that they no longer do testing with it to make sure that it's working with various operating systems. While it MAY work, they cannot guarantee that it will work.
So the question I present to you is have you done ANY updates to your system, your scanner, anything around the time that this change occurred?
One option for you that may allow you to continue working with Acro-9, is to not scan from within Acrobat. Keep in mind that Acrobat cannot do any scanning by itself, it links to your scanners software and you use that within Acrobat.
With that in mind, try using your scanner's software by itself. I do suggest that you save your pages as TIF images in a folder on your computer and when all scanning is done, than open up Acrobat, and process the images from that one folder. [Note: if you have (say) one 5 page document and one 2 page document, do them as two sets. Otherwise Acrobat will combine them all into one document. Not a horrible situation but that will just add an extra step or two for you to finish your work.]
And, as long as I have your attention, you might find some value in this blog I wrote for Adobe on how to get a clean scan.
https://forums.adobe.com/community/creativepipeline/blog/2018/01/22/scanning-clean-search-able-pdfs
Lastly, if you still have problems, make sure that your scanner's software is up-to-date and if you are still having problems, time to update your stuff.
The sad reality of software is that it needs to balance other things you have. As you get new hardware, operating systems, individual programs, they all have to know how to work with each other. There's no way for new software that was never tested against old software to reliably work with each other.
Let us know how things work out.
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