Hey there!
Ok, so there are 2 issues here. The UE4 issue is luckily a super easy fix. When you import your FBX into the engine (you’ll want to import and not reimport to get this option), check on Use T0 as ref pose. This will fix your bind pose mis-match issue.
As for the lost model pose and the subsequent errors, that’s a bit harder to debug off the current information. The model and rig pose are stored under a group node named Skeleton_Settings. If you do not have that node, that would be the source of the problem.
(to see this node and its children, you need to go to the Display menu in the outliner, then advanced asset contents, then “both”)
As you can see in the image, the model and rig poses are stored on their own nodes under skeleton settings, so if those nodes don’t exist, (accidentally deleted?) then you would run into this.
Without the full stack trace or a view of the file, I can’t be sure what the error you are running into is, but I think it is from the function that is trying to cache the weights when you go backward. If I had the full stack trace, I may be able to help more there. If you have the latest files from dropbox, that specific line is trying to open a file to write to. That file would be in the MayaTools folder, General\ART\system\ + geo name + “.txt”. If a file like that already exists, but is not writable, that error would make sense.
If you can get it to go backward, then it’s really easy to sort this all out and get your poses back. Check to make sure you can write to the directory and/or file it is trying to cache the weights to.