Based on this question - see quotes there:
In earlier Canon version (The Dragons of Spring Dawning), Berem unwillingly opened the gateway planted by Takhisis in the Foundation stone, allowing her passage, but due to Jasla's death, Takhisis could not enter Krynn completely.
In the later Canon version (Dragons Of The Hourglass Mage), the gateway was already open by Takhisis, but due to Berem's interference and Jasla's death, the gateway became partially blocked, allowing Takhisis to enter only as a shadow.
Why did the authors make such a contradiction/retconning? Is there any quote or interview clarifying this? Speculations are ok, but I would prefer a canon source.