One of these customizations is a simple function run when the document is first loaded that checks a couple attributes of the root note:
var actDoc = Application.activeDocument;
var docRoot = actDoc.getDocumentElement();
var rootName = "" + docRoot.getTagName();
Right now, docRoot is returning null which breaks the getTagName() call. Other exceptions in some of our other custom ACL also say:
Wrapped com.arbortext.epic.AclException: [A12653] The "detail" command is illegal for a text file.
There were quite a few changes in behaviour between 5.3 and 5.4 (but less changes between 5.4 and 6.0).
Upgrading Arbortext Editor is only easy in cases where you have no customisations. In cases where the Editor environment is heavily customised then you often need to engage the services of an Arbortext expert. If you feel like becoming an Arbortext expert then you will need to at least have taken the basic training, and for your particular task I would recommend studying the 5.4 release notes and 6.0 release notes to get an idea of exactly what has changed and what is new.
Next steps would then be to study the customisations and understand what is being added to the Arbortext environment and why. That will then lead you to understand how to replicate those customisations in 6.0 (in some cases, customisations can even be removed due to features which have been added in 6.0).
In extreme cases it can take weeks or even months to correctly migrate from an old release to a new release of Arbortext. It really depends on the level of customisations and the version gap. BTW, there are a number of organisations specialising in Arbortext services, one of which is the company I work for (GPSL).
Sorry I can't be of more assistance but it sounds like someone will need to debug the customisation code.