gradient mesh shortkey doesn't work

'Shift+C' is not the "official" shortcut for the meshtool (it does not yet have a default keyboard shortcut assigned).

'Shift+C' is also an already used keyboard shortcut (internally hard-coded, not configurable for users)[1] in the node tool context to convert selected nodes to cusp nodes. I would recommend to define a unique custom keyboard shortcut (e.g. like 'Alt+M' or 'Shift+M') to the meshtool instead for now.

[1] https://bazaar.launchpad.net/~inkscape.dev/inkscape/0.92.x/view/head:/src/ui/tool/multi-path-manipulator.cpp#L699

Probably set it by accident if it wasn't "hard coded".

Tried to changed it before in the preferences but it didn't seem to work.
Going to interface/keyboard shortcuts/context/mesh, just realised it takes seconds before the add new accelerator updates.
Got set it to Alt+M then.

After testing it with Alt+M it works!

Update:

having set Alt+M in a win64 build Inkscape 0.92.0 r15299 pressing the shortkey doesn't bring up the mesh tool every time. Not sure what triggers the behave, but sometimes nothing else helps getting to that tool than going back to the preferences/interface/key shortcuts/context menu and set the exact same key combination-or else- again, so that when the new key combination is set the tool is chosen automatically.

Comment 3 relates to a conflict with (translation-dependent) mnemonics in a currently focused docked dialog (in this case, with English as UI language, with 'Object > Transform > _Move'), as described with other keyboard shortcuts e.g. in bug #768103 and bug #1024277.

Shortkey failing has something to do with docked dialogs in focus.
[quote=su_v @irc]
If a docked dialog has focus, and defines a mnemonic with that letter, it will override a keyboard shortcut. moving focus to canvas helps.
Tracked in earlier reports.[/quote]