The debuglog is a very useful Tool if you have a problem with a Mod or if the Combination of multiple Mods doesnt work as desired. Here a Manual on how to use it.

Activating the Debuglog

First the Debuglog has to be activated. To do this make a right click on X Rebirth in Steam and select the bottommost Point "Properties" in the Context Menu. 
In the next Window you now select [Set Launch Options] and enter
 -logfile debuglog.txt
into the following Window and finally confirm this with [OK] and [CLOSE].

Creating the Logfile

Depending on what kind of Error you have it makes sens to let the Game run longer or shorter. Often it is enough to start up to the Main Menu and then exit for a useful Log, in case of more tricky Bugs the Game should be played until the Error occurs and then closed immediately. Also keep in mind that the Debuglog is created anew with every Game start and the old one is deleted.

Locating the Logfile

The logfile is located in one of the numbered sub-folders of
%USERPROFILE%\Documents\Egosoft\X Rebirth
(you can simply copy the Path into the Adress Bar of a Folder Window ;)  )
every of these numbered Sub-Folders is for another Steam-Accound used with this Windows-Useraccount, usually there should be only one here.
For Mod-Users: Open the contained file debuglog.txt and, if its a short file, copy the contents in your Post. When the logfiles are long i suggest to upload them to a file hoster to avoind unnecesary scrolling through the Topic because logfiles can get very long, depending on the quality of the used Mods.

Interpret Logfiles

Most stuff here is self-Explaining, therefore i only make a List of the confusing things:

 [=ERROR=] Cannot match path '/////////' in patch file '///////'. Skipping node.

This Error does not only mean that the XPath in the mentoined file could not be found but also that it resolves to more than one Node - XPaths must point to exactly one Location in diff files.

[=ERROR=] AI::AIDirector::LoadAIScript(): Ignoring root node 'diff' in XML file '/////////'. 

This "Error" can be safely ignored. X Rebirth tries to interpret xml-diff-files as actual AI/MD-Scripts (additionally to patching the original files) which can not work. not beatiful, but harmless.

[=ERROR=] GetText(pageid=////////, textid=///////) TextID not found!

The loopup for a Text doesnt work here. Best look through the english Language file (File Name t/0001-L044.xml ) or, if not present, the fallback Language File (t/0001.xml) for the mentoined Page ID and check if the text/t ID is present.