Your problem is not obviously caused by any supposed "instability" of GSX 2.6, which is far more stable than 2.5 ever was.
The error in your log seems to be caused by something that shouldn't simply happen, since it says GSX cannot load an object which is SURELY included in the installer so, it's not possible this can be an installer problem.
What might have happened, is that something in your system has prevented the installer from updating the add-on.xml file with the path to the new PBR objects, which is really the only possible explanation for not being able to load that object, which is the most important of the whole GSX: the default replacement jetway so, you can be sure it's there.
Some possible reasons for the add-on.xml not being updated are:
- the file became read-only for some reason. We surely don't set it like that, so it must have gone read-only because of some other program. Another user reported a similar problem.
- the file went was corrupted on your system, so our installer couldn't update it (it's not possible to update a corrupted xml )
If you could post your add-on.xml file located here:
Documents\Prepar3d v4 Addons\Fsdreamteam\Addon Manager\add-on.xml
I could check it if it's corrupted, but only you can verify it it's read-only on your system. DO NOT post the file content. ZIP it first, and Attach it to a post.