Validation error on saving modular

Edit: I mixed up the versions below. This is now corrected.

I started my Grav instance with version 1.6.31 and I created a custom theme based on Quark 2.0.3. My custom theme extends some parts of Quark and includes a modular to display blog items on the main page. All worked fine until "recently". After some updates, I'm now at Grav 1.7.17.

Now here is my issue. When I try to save my blog modular from the admin panel, I get an error:

Failed to save entry: Validation failed:
Invalid input in 'Folder Name'"

Point is that I have no idea which "folder name" is meant by this. I already tried to strip the modular to its bare minimum, yet this did not help.

Any suggestion where to look or where to start is greatly appreciated!

Thanks for your help!

The linter did not report any errors. So I created a local copy and stripped it down, as you suggested. And as you suspected, it was the custom blueprint for the blog module. After cleaning it up, the error is gone now.

Glad I was able to assist and get you back working :)