In Joomla 2.5.10, they have modified the Joomla installer and created a regression issue that we have reported to Joomla.
If you want to install a JMS 1.2.97 on a J2.5.10 or higher, you need to rollback manually a change that they did in joomla.
See Joomla bugtracker 30206
joomlacode.org/gf/project/joomla/tracker...racker_item_id=30206
We have fixed that in the JMS 1.3.10 and rollback (discard) the change done by Joomla.
As JMS 1.2.x is no more updated, you will need to rollback the update manually.
Otherwise, this should theorically work on pre-installed JMS 1.2.x on a J2.5.9 or lower.