Hi Bill@VT, Rave, and Sabian Zildjian,
Thanks very much for your suggestions.
I deleted ‘Acrobat’ and ‘com.adobe.Acrobat.Pro’ folders from Cache - no result; downloaded JAMF 'com.adobe.acrobat.90.sh.manifest', but couldn't open it (for Windows?); I also tried Keychain First Aid - no result (no problems found), and Repaired Disk Permissions - also no result.
I then slogged through all the incremental updates from 9.0.0 to 9.5.5... and the problem has gone.
Regards, Thomas