Interesting; thanks for the update, Chad. I have a bit more context on this error message I can provide you with, which may help with troubleshooting.
AtoM has a shortcut that allows you to preview your records in a different metadata template without manually changing the default template, by adding a semi-colon and the short name of the related standard (e.g. isad, rad, dacs, dc, mods) to the end of the slug.
For example, if your ISAD description is at:
Then you can preview it as Dublin Core by changing the URL to:
You can see this illustrated in slides 9-10 of the following slide deck:
The error you are seeing is from the routing validator, which is trying to parse the content after the semicolon as a metadata standard.
In your case, this is not happening for some reason, which is what led to me making the suggestions in my previous post. I did run a quick test, importing a record using the title shown in your example, and another one with no space before the semicolon and following text. I also created records with semicolons in the title via the user interface. I performed both tests with the URI path setting both on and off, and in all cases it worked as expected for me in our development environment. This suggests to me that, unless there's a regression specifically in 2.6.x that we've already unknowingly resolved in the upcoming 2.7 release, then the problem is likely local, and may have to do with the CSV metadata itself. I didn't test on 2.5, so it's possible this is an older issue that has been resolved in 2.6, but I couldn't find any specific tickets relating to this in our issue tracker - but if you found the issue after upgrading to 2.6.4, then that seems unlikely as well.
Did the import work as expected in 2.5 after you rolled back? If yes, then one workaround for now would be to finish your imports in 2.5, make a database backup, and then perform the upgrade. Doesn't help us identify and resolve the issue, but it should bypass the problem you're encountering for now.
Another way you can end up with non-standard content in your CSV is if you've cut and pasted content into the CSV from a non UTF-8 source, such as a Word document.