Menu editor shows error on saving

On saving the menu, an error dialog is displayed.
Using Gantry 5 together with Myriad theme

500 Internal Server Error
Oops, Cannot move files to existing folder/file..

File permissions for mainmenu.yaml are correct (0644) in directory user/data/gantry5/themes/g5_myriad/config/menu

In Grav, I can also only add particles to the menu, but I already found that the menu editor is limited for Grav.

How to proceed? I need to add additional menu items, and manual file editong is not an option, since I’d need to add an icon and dropdown (for language selection).