creating a directory yields "the file is to large for the filesystem"

My Windows 10 Pro (1803) seems to go nuts again (and me soon, too):

In a specific folder on the C-drive, let's call it "C:\foobar" creating a new subfolder always yields me an error message "The file is too large for the destination file system". What is this BS supposed to mean? First: it's not a file but a directory and why on earth can this possibly be too big???

My disk is NTFS-formatted, has about 1TB of space of which ~270GB are still free and I can't create a 0-byte file in that folder??? The name doesn't matter, some "real" name or just "x" all yields the same.

BTW: I already ran chkdsk of the drive and the ubiquitous "sfc /scannow" that MS folks always come up with as a cure-all (of course as usual without any errors...), so please at least spare me those suggestions!

The folder is called "C:\devsbb" and it is not any special folder but one, where I keep stuff that I am developing for my employer (and I am owner of that folder and I *do* have write permission there).

Re. the question: file or folder: the issue was with folders AND files.

Meanwhile things have normalized somewhat. I was able to create a bunch of new subdirectories and copy files into that directory and some subdirectories using a copy-utility (FastCopy). And for some reason I can now also create and copy folders and files in Explorer again.

However, RENAMING a subfolder of that folder or a file inside any of these folders still doesn't work! As soon as I want to change a name there, I am getting the message "The file is too large for the destination file system". That's why another sync utility I am normally using does not work anymore here: it first creates temp. names and only assigns the final name after transfer completion (and that renaming then always fails). Really weird behavior!

Only while I am running it manually - no background sync here. That's why I started investigating. That sync suddenly failed...

"chkdsk c:" in an admin shell. This runs chkdsk in read-only mode but since it reported:

Windows has scanned the file system and found no problems.
No further action is required.

… that should be good enough, I'ld say.

Thanks for responding and trying to help!
Meanwhile I boiled this down to a different issue and thus created a new thread here.