First, I suggest to download latest version of UltraEdit and run the installer to update to latest version. UE v21.00.0.1030 is not the latest version.
Second, if the problem exists also in latest version, I suggest to open View - Themes - Manage Themes while UltraEdit is on secondary monitor and select the theme Classic. Just selecting is enough, do not click on button Apply or OK. Now verify if the scroll bars of the view are visible and working. You can do that although the Manage Themes dialog is still opened.
Reason for this check: With theme Classic there is on tab Application the application style UltraEdit Classic selected. And with any application style other than Custom the scroll bars are not custom scroll bars drawn completely by UltraEdit, but the Windows theme based scroll bars.
If the Windows standard scroll bars are displayed and working fine on secondary monitor, the problem must be searched in code of UltraEdit for custom scroll bars.
Press Cancel in Manage Themes dialog to keep your preferred theme unchanged and report the problem to IDM support by email.
Important would be perhaps also if your secondary monitor is positioned left or right of primary monitor. All x-coordinates are negative with a secondary monitor positioned left of primary monitor as top left corner on primary monitor has always the x/y-coordinates 0, 0. I have seen many applications fail to display something correct on a secondary monitor positioned left of primary monitor because of using internally unsigned integers instead of signed integers for calculating positions of elements or handling negative values for x- or y- coordinates as invalid values although that is wrong with a monitor positioned left of primary monitor (all x-coordinates negative) or above a primary monitor (all y-coordinates negative).
Problem with usage of custom scroll bars with using a secondary monitor left or right to primary monitor is fixed in UE v21.00.0.1033 which is available for download from main download page. The problem with secondary monitor above primary monitor is not yet fixed, but the bug is confirmed already by IDM and should be fixed in next public release.
Best regards from an UC/UE/UES for Windows user from Austria