Since installing OS 4.2.1 on my iPad and installing 10 more volumes I noticed that sometimes when I change pages or change volumes I wind up with blank screens. I can't reproduce it on demand but it has happened a lot today. I don't know if it is because of all the new volumes I have added today or if it is related to the new OS 4.2.1 update from last week. Has anybody else noticed this?

Please try rebooting the iPad (hold down the top button and the home button and keep holding both buttons until the Apple logo appears, then release both buttons) and see if this issue goes away (at least for a long while.)

I did that last night but I am experiencing the problem again tonight. In NASS I have a blank Gen chap 50. I came out of a word search and then started seeing the problem. I have changed to ESV and that is also blank. I have backed up several chapters and those are all blank. Same with any version I have. Exiting the program and restarting the program does not help.
I just tried the tap to update in the bible section and that kicked me out of Mantis.
This is frustrating. I see the note on the Mantis web page that 4.9.2 has gone to iTunes but it doesn't specifically mention my problem.

Restarting the app will work for the blank screen problem, but you're not really doing that. In order to restart the app, you need to:

Tap the home button
Double-tap the home button
Press and hold on the Mantis icon in the multitasking tray
Tap the red (-) to "kill" the running Mantis app
Tap the home button
Start Mantis again from the home screen

This forces Mantis to restart.

Alternatively, you can just reboot (hold down the top button and the home button and keep holding both buttons until the Apple logo appears, then release both buttons.) This is the preferable way to solve it, since it clears as much memory as possible.

This does not solve the crash on "Check for Updates" -- that fix should come in 4.9.2. Reloading Mantis from scratch may also solve that problem, depending on your configuration.

The "white screen" bug is not yet completely resolved in 4.9.2, since it's something we can't reproduce. It's related to low-memory situations on the device, and is caused by a bug in the operating system. We are still working on this one.

Like I said I have already reset the iPad (just as you described) and it still reoccurred. I have reset it again just now and will be using Mantis tonight so we will see. I do appreciate your replies and the attempt to correct this bug.
I have been testing medical software for 18 years and can understand the complexity of different issues that arise. The unable to reproduce ones are the worst.
Any idea how long before 4.9.2 comes out?
Thanks

We're at Apple's mercy as far as release times go. In general, Mantis releases take at least a week, sometimes two. This is longer than most apps, from what I can tell, probably because Mantis is so much more complex.

Rebooting should solve the problem for you, just not permanently. When you get back down to a very low memory situation, it can show up again. For some people, depending on their habits with the software, it can occur frequently. Others never see the problem at all.