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Alternatively there is a setting in the advanced page of the admin portal. The setting is called Reverse proxy prefix. Yours should be set to say ubooquity. If you change this field to instead be blank your url would become Which is what the code in this theme mod is expecting.

Yes, I had to change the spots where comicid[2] appears in the code to  comicid[3] in order to get it to work correctly because this is assuming the structure of your URL. There is a function in the Comixology 2 Theme that could be used to make this mod work for both types of URLs. I would post the code but I added this these buttons and the progress bar to the other theme. In doing so I have made a lot more changes than just the one that you need.