I'm sorry I haven't been able to respond earlier, but now I have some time again to investigate your problem.
Assuming you are using a Duet WiFi, can you confirm you are using my latest Duet WiFi Server version 1.03? There was an issue in prior versions that caused empty responses to be sent to the web interface and this issue looks exactly like it.
Although I doubt an older version will resolve this problem, you can actually get older versions from my GitHub repository by looking through the list of commits. Here a link to the 1.12 DWC files, but be aware that it may not work properly with the latest firmware version: https://github.com/chrishamm/DuetWebControl/commit/edf9c9ba3f1148d67fbafeae0d6b20042705cb2a
0 chrishamm committed to chrishamm/DuetWebControl Packages for first-gen Duets and Duet WiFi added