Dear Paul,
thanks for trying to help me. I tried "modules/Offcanvas/ select hide ->then save -> Show then save" again, as suggested by Rashida, cleared all cache, tried in Firefox/Chrome/Edge - no change in offcanvas. Would you please have a look and click at the arrow under menu item "topic". There are 3 submenus listed (Instruments/Projects/CoreIssues) and each has its own submenu. If you click on the given arrow next to the three mentioned submenus, only the third submenu (CoreIssues) opens. But for "Instruments" and "Projects" the arrows to open submenu are missing. Any idea how to solve the issue?