As far as I know, the Jlink Ultra+ should be supported now. I have a Jlink Pro and I can program and debug through it. I am trying to buy an Ultra+ to test with, but in the meantime can you make sure adapter pack logging ([Window] > [Preferences] > [SimplicityStudio] > [Adapter Packs] "Adapter pack log") and device manager logging ([Window] > [Preferences] > [SimplicityStudio] > [Device Manager] "Log device manager activity to file at") are enabled and then connect the JLink Ultra+, wait a few seconds and then generate the Simplicity Studio log files (Please generate a zip file of the Simplicity Studio Logs - [Help] > [Report Bug...] make sure "Export Simplicity Studio log files" is selected, then hit [Next] twice and [Finish]) and upload the zip file to this Community Forum topic?
The J-Link Ultra+ is detected but there is a problem getting the debug mode and detecting the adapter / device. Is the board being used a custom board or a Silicon Labs STK? Do you have any Silicon Labs development boards you could try to attach the J-Link Ultra+ to try and isolate the issue?
I am using one of our production boards (EFM32GG330F1024) which both my Flasher and an STK3700 allow me Flash program.
The STK3700 gives the same result as the Production board. I am using SWD not JTAG.