I faced a similar issue with the iOS In-House app not installing using the OTA method. Here’s how I resolved it after burning precious hours of my life ;-):
Ensure the Dropbox links are modified to force downloads. Change the dl=0 to dl=1 in the links.
E.g., https://www.dropbox.com/scl/fi/.../YOUR_APP.ipa?rlkey=...&dl=1
Ensure that the second image in your manifest.plist is precisely 1024x1024 pixels.
The link to your manifest.plist in the itms-services URL must be URL encoded.
E.g., itms-services://?action=download-manifest&url=https%3A%2F%2Fwww.dropbox.com%2Fscl%2Ffi%2F...%2Fmanifest.plist%3Frlkey%3D...%26dl%3D1
After these adjustments, I created a QR code for the link. Scanning the QR code with the camera app successfully installed the app on registered devices (UDID) using Ad Hoc distribution.
st-gr
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