I found another bug. When I attempt to build a lock with out enough money, the error popup says Can't clear land rather than can't build lock. It only happens if I try to build the lock on a the slope in between two flat tiles. Do we consider that a bug?
Normally, when constructing anything, clearing of the land happens first, followed by the construction of the 'object'. Most probably, you did not have enough money to clear the land first. If you did, then, should you not have enough further funds to construct the lock, you get the correct message. I don't think it is a serious bug, not enough funds is indicated either way.
OpenTTD deals with the error of building a lock on flat or double slopped land with the error "Land sloped in wrong direction" and only allows for a lock to be built by clicking on a valid slope.
OpenTTD has seperate tools for canals and locks, whereas TTDPatch uses the same tool. You cannot really compare the two here.
... 'Can't clear this area ...
Clearing the area, before building the 'shiplift' !! happens in procedure cleararea in enhgui,asm line 1659 and there is your error too, in case something happened that prevented the clearing. This is a global procedure, most probably used by many construction events. I wouldn't touch it.