We did a migration on this site from 2.5.x to 3.x. Initially, everything worked OK. However, now any K2 link that is clicked - Add New, Edit, etc - results in a white screen. Page source show:

<div id="k2AdminContainer" class="K2AdminViewItem isJ25 isJ30">

Turning on Development error reporting shows nothing.

Also, unable to edit from the frontend.

Hello,

Please enable error reporting so we can see the actual error.

Please refer above... "Turning on Development error reporting shows nothing."

How about the error_log.

Only one line in it, and that from yesterday (the error with K2 has been going on much longer than that)'

[16-Feb-2017 09:09:48 America/New_York] PHP Fatal error: Out of memory (allocated 4980736) (tried to allocate 49152 bytes) in /home/caltaste/public_html/plugins/system/admintools/util/exceptionshandler.php on line 868

This is an Admin Tools related issue.
Try increasing the memory limit.

Are you using any JS scripts which alters the markup?

Not sure - nothing to different from his other site which is built pretty similar. I went to a K2 page (front end) and checked the script calls - these are what's listed:

<script src="/media/jui/js/jquery.min.js" type="text/javascript"></script>
<script src="/media/jui/js/jquery-noconflict.js" type="text/javascript"></script>
<script src="/media/jui/js/jquery-migrate.min.js" type="text/javascript"></script>
<script src="www.tastecaliforniatravel.com/media/juloalib/js/jquery.min.js">
<script src="www.tastecaliforniatravel.com/media/juloalib/js/fullnoconflict.js">
<script src="/components/com_jrealtimeanalytics/js/jquery.noconflict.js" type="text/javascript"></script>
<script src="/components/com_jrealtimeanalytics/js/stream.js" type="text/javascript" defer="defer"></script>
<script src="/components/com_jrealtimeanalytics/js/heatmap.js" type="text/javascript" defer="defer"></script>
<script src="/media/k2/assets/js/k2.frontend.js?v2.7.1&amp;sitepath=/" type="text/javascript"></script>
<script src="/media/jui/js/bootstrap.min.js" type="text/javascript"></script>
<script src="/media/system/js/html5fallback.js" type="text/javascript"></script>
<script type="text/javascript">

Can you try disabling extensions one by one in order to see which is causing the issue.

Furthermore if you have a dev environment, that would be ideal.