f6aca74944482c1bf5f2df653945a2fb5d2bf660 has strange side effect. I bisected locally and revert ontop 1.11.5 fix my issue. Any 1.11.* (on Debian) fail to do from command line: I didn't try 1.12 because of dependency issues. `kdiff3 a b c -o d` Doing the same from UI works fine, but command line invocation always fail with "File open error - KDIFF3" dialog - b Opening b failed. Unknown error.
1.11.5 is a missing one patch the came in after the switch to 1.12. The fix is at https://invent.kde.org/sdk/kdiff3/-/commit/09b2adaa9e4dc89d02d6cff1e387408d5c5e2dbe it arrived just ahead the switch to 1.12 branch for releases. It should apply cleanly to 1.11.5.
*** This bug has been marked as a duplicate of bug 486782 ***