Actually to be more specific, the error appears when previewing site:start and not when editing.

I can't work out what this would be, or reproduce.

My best guess is it is lang cache corruption when a sitemap is generated, and a delayed display of the menu.

Let me know if it reproduces after clearing the language cache.

I am using the <tempcode params="{+START,IF,{$NOT,{$IS_GUEST}}}">{$LOAD_PAGE,start,site}</tempcode> code Malatesa posted. However, I made that change 2 days ago, and this just appeared today.

(Click to enlarge)

Gone again now, but it was there for quite a while.

Is there anything interesting you can point to for this page, e.g. something catalogue related?

Oh wait, in the screenshot, some kind of catalogue entries block.

Fixed in https://github.com/ocproducts/composr/commit/4f62d64f00fb1c4ec09dc9579df31599297d7010

I could not spot before as there's some dynamism in picking of the language strings, a code search missed it

Made that change, emptied template cache, still seeing the missing string message though, on and off.

Empty block cache also.

It just disappeared, but will do that now.