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!
Hi @macrospoerl
Are you using any custom blueprints for your modules. Have you checked there is no invalid / errors by using the command line
php bin/grav yamlinter
I would then create a test server / local copy and remove all your pages which your modules user (ie copy them out the directory) and see if your create a clean blog modular page item, does it work, this will show if it is more to do with grav / your theme, if it works then it is probably some inconsitency on your existing pages
This would be your start point, then we can go from there. 
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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.
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Glad I was able to assist and get you back working
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