UFT 12.50 now has the option to return Run Results as either an HTML Report or Run Results Viewer Report:
UFT > Tools > Options > General > Run Sessions > Report Format
The default is set to HTML Report. When running tests from ALM/QC, you can view the HTML report if you are using one of the following ALM/QC versions: ALM/QC 11.52 Patch 5 or higher, ALM/QC 12.21 and higher (from the UFT 12.50 ReadMe).
So, if you set the Report Format to Run Results Viewer Report, then your QC 12.20 instance will be able to open the test run results. For test runs where the test instance was set to HTML Report format, you will receive the Add-in error. If you update your QC server to 12.21, you will be able to view both HTML and RRV report formats for tests run from QC.