Word crashes when Add-In attaches a template

No one here can tell you what has changed since 2606.

Your title talks about attaching a template. Your text talks about loading one. These are very different functions.

Attaching means it becomes the attached template for the document. Macros and other resources in the template are available in the document. the Attached Document Template (https://www.addbalance.com/usersguide/templates.htm#attached)

Loading means the template is added to the Add-Ins collection in the Word application and is available in all documents. Global Template (https://www.addbalance.com/usersguide/templates.htm#Global)

From the message you get when restarting Word, my guess is that you are loading it as an Add-In.

As a workaround, consider doing what you currently are not. I.e., load if currently attaching and vice versa.