I also get the exact same error on my Shiny app based on Golem using MacOS ventura. It seems to be an erratic behavior. 90% of the times I try to deploy this happens, the other 10% it goes through without any change other than just restarting the session. It is very annoying and time consuming. Google searches suggest to remove the .Rhistory files and check for non-ASII characters, I've done that and sometimes works but it never goes totally away. I hope someone can give a better answer than me.
ASCII is a character encoding with only 128 code points, the first 32 of which are control characters (e.g. tab, newline). 0x8b in decimal is 139, which is too high (i.e. > 127) if something is trying to decode as ASCII. Until about 20 years ago it (or variations of it in the form of ISO-8859) was the predominant encoding used by computers in western locales.
But your error is about UTF-8, which is an encoding that's designed to be able to handle the whole Unicode character space while having the first 128 code points be backward compatible with ASCII. UTF-8 uses a variable number of bytes per character, but not every possible byte can be the first byte of a character. Clearly 0x8b is one that can't be.
The TL;DR is that somehow you're getting an invalid character in your output. I don't think it's diagnosable based on just the error message.
What would be helpful would be a reproducible example (aka reprex) of code that triggers the error. I know it can be hard if the bug happens intermittently, but if it's even fairly consistent, then try to make a minimal example off your code that gets the same behavior.