Great news! I have just discovered the solution. The problem was because all of the models I was trying to load were within a folder names "Figures, Dioramas, and Busts". The commas ( , ) within the folder name were causing the File does not exist! errors. Once I renamed the folder to "Figures Dioramas and Busts" without the commas between the words the STLs are now opening successfully in Chitubox.
Perhaps this is something that you can either include a warning about within the program or a proper rule-set for where the STL files are kept within Windows. Or possibly in the next update to Chitubox you can ensure this doesn't happen to files contained within folders that include commas or other effected punctuation.