[imp] Search results of more than 100 messages cause 'error communicating with the server'

[Tim ODriscoll]

Dear All,

I've recently upgraded to Horde Webmail 5.2.1, with IMP 6.2.1.

When any of my users try searching for messages in a mailbox and the  
result returns more than 100 results, they get the error above: error  
communicating with the server.

In the Apache logs I see this:

PHP Fatal error:  Call to a member function fetch() on a non-object in  
/usr/share/horde/imp/lib/Contents.php on line 1616, referer:  
https://mydomain.com/horde/imp/dynamic.php?page=mailbox

If a search yeilds less than 100 results, then another search  
immediately afterwards returns more than 100, there is no error when a  
message is clicked on but the message contents shown are from the  
previous results list.

Any idea where I can start searching to fix this?

Many thanks,

Tim