Welcome to the Magix Sound Forge users community @Joseph-Boor, Which version of Sound Forge are you referring to? SF Audio Studio 17 is sluggish opening the MP3 metadata ID window, but is does open after clicking it a few times. I often use the third-party "MP3tag" editor. A cover photo can be added with that as well.
As you are probably aware, some media formats inherently do not support user metadata.
I am not familiar with "version 26.0.0.23 (DP3)" Are you referring to the MP3 metadata using Sound Forge? Audio Studio or Pro). There is no Sound Forge version 26 that I am aware of.
Otherwise, try resetting Sound Forge and clearing the cache. 'File> "Reset all preferences and clear cached data" This resets most of your other custom settings to the defaults as well.
To avoid confusion, always indicate the full name of the program. There is no product called ID3 Editor and there are 3 programs that start with the name Sound Forge. The current version of Sound Forge Audio Cleaning Lab 4 is 26.0.0.23, so I assume that that is what you have.
In this program, same version, the ID3 editor works fine for me. Use the shortcut L.
Create a simple project with 1 audio object on the arranger. Press L. Does the ID3 Editor open?
If not, what is the type of file on the arranger?
If your computer and operating system meet the requirements indicated by Magix, then I suggest that you open a ticket with Magix.
I see now the product in question is "Sound Forge Cleaning Lab 4 build 26" and the topic should have and been placed in the Cleaning Lab users forum to begin with. This is common however and Magix should have named it differently IMO. Sound Forge 'Pro' and 'Audio Studio' both have basically the same UI and are very similar otherwise, OTOH, Cleaning Lab is a different animal with only a few similarities to SFP, SFAS and has a much different GUI.
I will move this topic to the appropriate forum to avoid more confusion.