Buildcraft version: 7.99.7
Forge version: 13.20.1.2429
Link to crashreport or log (on pastebin or gist, please don't put the entire log here) :
Singleplayer/multiplayer: Don't know if you need it yet.

Additional information:
The tool tip for the Creative engine in earlier versions implied that it was a beast, and a quick Google search reveals that is should be, but does about the same work as a red-stone engine (I think). I was going to put the red-stone engine on the quarry for comparison, but you can't power quarries with a red-stone engine.

After some digging in i discovered that the TileEngineCreative.onActivated method, that is in charge of changing the Creative Engine speed, isn't being called when sneaking with an item in your off hand.
After going dipper i discovered that the BlockEngineBase_BC8.onBlockActivated method, which is in charge of calling TileEngineCreative.onActivated, isn't being called when sneaking with an item in your off hand aswell.
When i checked where TileEngineCreative.onActivated is being used i saw that this method is only being called by the Forge API, therefor this bug is not is not caused by Buildcraft but by the Forge API.
The only solution i see at the moment is waiting for Forge to fix this.

correct this has been a know forge bug for a while. waiting for forge itself to fix it can take ages, basicaly until someone submits a PR to fix it

Would be difficult to fix I suppose, as there are mechanics allowing (and somewhat requiring!) you to use item from the off-hand.
Example: I use Morph-O-Tool to configure Buildcraft pipes, but it doesn't morph into Buildcraft wrench while looking at pipes, only at some engine. Therefore I have to look at the engine, press F to move the wrench off-hand and then "use" an empty hand on pipes. Then the wrench'es activation handler kicks in and the wrench works.
If they remove off-hand activation or even change the priority of activations, it could break certain things severely.

@ogurets if the morph-o-tool doesn't morph when looking at pipes that's probably because using the wrench on pipes doesn't do anything

also this issue is not about removing or changing any orders, just about off hand activations not being triggered when they should by forge

Tried that, didn't work. I have rebound crouch to Ctrl and did a Ctrl + click.

Shift + click it with wrench.

For the record, I tried Shift + click as well. That didn't work either.

And by click, I mean Right Click (I assume you meant Right Click. Left Click destroys the block)

Like this?

Then Ctrl + click works for me. Can you screenshot or record video?

Were you holding something in your off hand? I noticed that it did work after removing the torches from my left hand.

No, I weren't. And yes, many mods have this bug.