Error detected while processing function <SNR>20_opendir:

[mtglsk]

It seems to get triggered when a buffer is modified and you hit -.

[mtglsk]

#45

It seems that on Windows Illegal filename shows.

On Linux I am getting the expected E37 (No write since last change). Not sure if this is the same issue.

Still it says Line 13 of the function, so maybe it's triggered the same way. Here is a simple way to fix (I'm not sure if it could do any different when nohidden is set?):
Line 49:
- execute a:cmd '%:h/'
+ execute 'new %:h/'

Edit: This doesn't seem to work with an empty new buffer, only with opened files.