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Thank you for the video, you can easily fix this issue.

never mind .. i will check then in next build ;)

-Validated-

Whoops, just saw this on the main list, forgot to check if it was in queue. It was a really simple fix, but I might have messed up the workflow for this issue.

-validated-

I was unable to reproduce this. Pressing the num pad keys while in build mode with only a core block had no effect on my client.

Intended, duplicate of T1043
Not an exploit, creative mode gives you infinite blocks anyway and you can only get this mode through admin commands.
The "creative" mode for shipyard designs has some of these things disabled to prevent normal people from exploiting it.

Appears to be related to player count. As soon as more than 1 person is online, the crashes happen much more often, making the server unplayable due to being unable to say connect for more than 5 minutes

I had this issue aswell a while ago (like months) but haven't been testing it again since.
My first guess after I read the comments here is that it might be cause by very slow CPUs in combination with playing on a server. (E.g.: I play on an i5-2410M @ 2.3GHz (not great but it works - kind of.))