The console error shows up here as well. Not related to failure to display UserCP icons, at least as far as I can see. UserCP icons display properly here, even with that error showing in Console.
So, a couple of questions come to mind.
1. Do other icons show up properly, like on the forum index page and elsewhere? It's only UserCP that icons do not show?
2. Do you have plugins that modify UserCP? What happens with all plugins disabled?
Otherwise, the only difference that I can see is that my browser is blocking the part in headerinclude which invokes googletagmanager. Consider editing the headerinclude template at the bottom to delete the two scripts which invoke it, although I doubt that would affect icon display, because the scripts are there in forumdisplay page, which I assume is working properly for you.
I edited out of my earlier reply some stuff that did not solve your problem.
One of these days when I have time, I will write a troubleshooting guide for css using easily accessible tools.
I'm not an expert, just an ordinary guy having fun.
Because I believe there's always an opportunity for a learning experience, and keeping in mind others who follow the problem, not the user, here's the short answer.
Use your browser tools. I'm most familiar with Firefox, and it allows you to select or deselect css attributes. Use the Element selection tool to find the specific element you want to troubleshoot. In this case it is ::before which invokes a fa attribute. If you deselect that element in css, the text portion slides to the left which proves the icon is there. Still looking at that icon formatting, you might see a color attribute which might override an earlier attribute. If you deselect the latest color, the underlying color shows up. There's your answer.
The icon is there, just formatted to the same color as the background. Browser tools are very powerful. I'm still learning how to use them.
A point needs to be made here. It would have been impossible for someone to diagnose this problem without your URL. I found it with a little searching. This could have been another question that was never answered or worse, one that was solved by the OP, but the solution was not shared for others to learn from. This is why I changed my answer from the snarky tidbit that was visible for a while.
A site owner can claim credit for his forum design, but if it's MyBB, it is "powered by MyBB." Hats off to the guys who work to advance it.