Hi Brian, can’t say this will work for you, but I did have some issues with custom reports in my testing. This was during the Controlled Release, so it may have been related to THAT specific environment and not necessarily applicable to an on-prem system… but…
My issue was my custom reports weren’t working at all… errors saying the reports could not be found, locations, blah blah blah.
Obvious next step was to take my custom reports from my LIVE (2025.2) environment and upload them into (2026.100). This also failed!
My existing custom report styles would not take an upload.
I found I was able to get around this by making a fresh copy of the BASE “Standard - SSRS” report style from within 2026.100. I then uploaded my zipped custom RDL(s) into that fresh copy. Repointed to my custom RDD (if necessary). I was then able to run my custom report.
Can’t say why that all was needed, but making a refresh custom report style WITHIN the 2026.100 environment worked. Using report styles created in previous version would not.
Again, just something you could try and see if you can get any further so you can continue testing.