Debugging software is not hard. There are four steps:
Deny that there is a problem.
Deny that it is your problem.
Ask for more information.
Complain.
Steps 1 and 2 are self-explanatory.
In the third step, ask for more information, such as what is wrong with the software and exactly how to fix it. Ideally every bug report sent to a developer should have a Pull Request with code that fixes the bug. This is how open source does it after all. If you are given an exact reproduction with all the information you need, move to Step 4.
If you must debug this software, then make sure before you get started to tell everyone how hard it is going to be, and how making you fix software rather than build new software somehow demeans and devalues you. Tell them that you were hired as an Architect and Do Not Do Support, but that you will help out this one time. If the CEO of Uber can learn something from working as an Uber driver, then you should be able to learn something from debugging the code that some inexperienced idiot (you, 6 months ago) wrote.
Then, if you run out of ideas, go to:
Step 5: Add more logging and return to Step 1.