Xodus copes with large numbers?

We've had Xodus up and running on a website fine. It has stopped working - to a degree.
For user groups with small numbers, it works ok - csv or Excel. If we select 'members' however, the 'saving' dialogue comes up but nothing happens. We know there used to be a limit in Xodus but this was solved some time ago.
Our thoughts are:
#It could be the user numbers have grown too big for Xodus to cope? 3500+ member users
#Some foreign characters are used in the Full Name field - possible corruption?
#It's still running on Modx 2.2.14, and Xodus was updated for 2.3. Is there a compatibility issue? The site's a bit of a beast so we've always shied away from the update...
Anyone point us in the right direction here?

Since posting, we tried a carbon copy of the website in a local hosting environment using Xampp just to rule out server issues. The problem is the same. Any ideas anyone?

Sorry to keep posting back to my own thread - more info:
A check on the modx_users table returns 'OK' so no apparent corruption.
Each tim eXodus is run, apache error logs show 'mod_fcgid: stderr: PHP Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 71 bytes) in /public_html/core/xpdo/xpdo.class.php on line 1752'
The php.ini shows Memory_Limit set to 128MB

I know little about this area, but I imagine 128MB should be enough and that there's some memory leak in Xodus when coping with a few thousand users?

For the record in case of future readers of this post with a similar problem, it was fixed in a half-hearted way by increasing the php.ini 'Memory_Limit' to 256MB. The Excel function still doesn't work but the CSV now does. So my guess is Xodus was built with smaller user numbers in mind but would welcome any input from others on how to scale this plugin.