If the file APPOSTR.xxx (in fact any file APPOST*.xxx) is not zero bytes, then rename it, relaunch Adagio AP and try again. If that doesn't work, I can guarantee you that if you have access to ACCPAC Plus AP and you try it there, it will work. ACCPAC Plus AP is not as finicky in some regards as Adagio AP.
May I take this opportunity to once again ask Softrak to lift the restriction on changing vendor account sets or bank codes while there are unpurged posting journals. I can understand the restriction for unPOSTed batches, but I could never understand the one for unPURGEd reports.