I just experienced this same problem after using Ledger 9 for 3 months without an issue. Knowing that the 'Posting complete' is next I just pressed the 'Enter' key. The next step was also hidden, which is to print the posting journal, so again I pressed the 'Enter' key to select the default option. When the posting journal finished printing the 'Purge' function was also hidden. Again pressing the 'Enter' key selected the default option and the 'Merge progress screen' finally went away.

Thankfully Adagio is completely keyboard driven.