Just had the same problem with tailgate not opening and tailgate open message and unable to lock car, tried all the methods mentioned. pushing bottom in lifting, pushing from inside but nothing would move it. I could hear the switch click but not the latch. I was then told by someone to press the "Lights On" button on the keyfob then while the lights were on operate the latch as usual, didn't think it would work but it did and tailgate opened as normal then when I shut tailgate the message had gone and car locked normally. The thing I have found is that if I try to open the tailgate as normal it won't budge but the error message and locking problem come back until I repeat the Lights On procedure. The other thing I have found is if I use this method when the car is locked the alarm goes off, so have to unlock first then open tailgate.
This has got me stumped as the switch and solenoid work as they should when this is done but not when the "Normal" method is used. Anyone got any ideas or the schematics of the electrics, or the Harry Potter book of magic spells.
Update on this, after a long think about it, if the switch and the solonoid work when Lights On pressed, then what is stopping them otherwise. After a bit of investigation I found that the towbar (Which was fitted when I bought it) is not using the dedicated wiring harness, it has been connected into the wiring somehow , have not removed all the insulation tape yet, and has a separate beeper/relay box. I suspect this is causing the problem and will go and see a towbar expert in the week. the wiring harness looks expensive though.