This is a current known issue caused by an Adobe bug.
The bug was introduced with the Adobe Dimension Tool in Illustrator 28.1, affecting all third party plugins, including ours. Luckily, it can be closed by simply closing and reopening Illustrator.
The problem occurs when you open Illustrator 28.1 and select the Dimension tool in the toolbar, the third party plugin icons then appear blank throughout that session.
The workaround is to Select Illustrator > Quit Illustrator (macOS) or File > Exit (Windows) and then reopen Illustrator.
At the time of writing in January 2024, we do not have a timeline for the solving of this issue, but you can read more about it here in the Adobe Forum.