If you are exporting a mesh with buggy buggy and you have a problem, it is suggested (from experience I've had) to close your program and relaunch it.
For your issue of it not being found, maybe your object (somehow) has gone back to being a Editable Poly/Polygon? It must be
Editable Mesh!Or perhaps, you are trying to select the Ghost object, Buggy buggy creates a duplicate of your model (to prevent damaging/destroying/ruining your work) and when an error occurs, it creates an additional object clone, as seen here.
You may not see it, but there is Haus001 now instead of Haus. Where is my Haus??It is there, but when you carefully select it, you will see that the backup (I made) as well as the original object is there, as well as the one made by buggy buggy.Or maybe your objects are frozen or hidden, perhaps you have to unhide them? I am not sure, as I can't tell if there is a difference between Gmax and 3ds max, they are almost the same but your UI is quite different to mine...
If nothing else works (or as Dreamk's suggestion) - My best advice is to (if you have a backup) start again from a latest backup. I always work and save as at different stages where I need to roll back.
Backups will SAVE YOU!
Also another thing, I replicated your error you had earlier - this occurs when you have a multimaterial or simply put, an object with polygons on a different material ID number as here in the photo:
The error
You can see one part of the house is off color, this is because if you look at the second photo..
The poly ID is 64, when you have only 3 possible choices in the multi material.
I highly suggest modders use a single texture (Standard, then Diffuse Color) with the SAME NAMES to create less confusion and for the ease of naming conventions of textures.
If you are making a single object/part for an aircraft which does not need to be added to the skin texture, (or even a simple building like this for example) I highly recommend using Ctrl+A (Select all) and making all the polygons a single Material ID (Since to be honest, it only needs a single texture - more complicated objects like aircraft fuselages/buildings/high detail buildings may use many more than just one texture so this is where multimaterial ID comes in use).
Press Ctrl+A (if there are blanks in those fields, that means the materials are not uniform/more than one material ID which will cause the issue above)Now all the polys are on Material ID 1 and now show and will not throw an error!Hope this helps.