Thank you for your response, Michael (and for the patch you have 
already committed). I cannot update the Horde/Turba installation 
myself, but whenever the IT company that manages the installation 
updates Horde/Turba to a more recent version, I'll confirm that the 
bug is fixed; in the unlikely event the bug persists, I'll reopen this 
bug. Thanks again!

This was already fixed. Please check that you are using the most up to 
date packages.

Since the Jun 6, 2022 commit to lib/Factory/Driver.php (and later, 
related commits), virtual address books (VABs) have been broken for a 
user who had the following pre-existing VABs (see the SQL dump below; 
these are very basic VABs, nothing fancy, and therefore probably in 
wide use); all the errors generate the log entry below (see horde.log) 
and display a plain HTML page with the same error. Some workflows that 
cause this error: (1) attempting to create a new VAB for search 
produces the error (though the VAB does, in fact, get created); (2) 
the existing VABs cannot be browsed or deleted (the final step, when 
'Delete' gets pressed, delivers the error); (3) many email messages 
(but not all) cannot be displayed when double-clicking on them (prob. 
because of a VAB lookup that fails).

Again, this doesn't happen for all users, only for a user who has 
pre-existing VABs, like the ones below. It appears the issue is 
simple: the 'source' below is a string, but the Jun 6, 2022 commit 
throws an error if 'source' is not an array. So maybe the fix may be 
that, when the turba_shares rows are loaded from SQL,' source's which 
are strings must be turned into arrays before getting passed on to 
Driver.php.

horde.log error:

2022-07-01T05:46:17-05:00 EMERG: HORDE [turba] $config must be an 
array [pid 976297 on line 55 of 
"/usr/local/cpanel/base/horde/turba/lib/Factory/Driver.php"]

SQL dump of relevant table rows (email replaced with 
'nobody@nobody.com' for privacy):

INSERT INTO "turba_shares" 
VALUES(922,'t27FW8VJbiEKw01sVBO6NF4','nobody@nobody.com',0,0,0,0,'vamvakas',NULL,'a:3:{s:4:"type";s:5:"vbook";s:6:"source";s:32:"564f76d6a9885108f812ed52325f595c";s:8:"criteria";a:0:{}}',NULL);
INSERT INTO "turba_shares" 
VALUES(923,'Qfa8ky2wYRfNUTqmM8T8R7o','nobody@nobody.com',0,0,0,0,'vamecos',NULL,'a:3:{s:4:"type";s:5:"vbook";s:6:"source";s:10:"favourites";s:8:"criteria";a:0:{}}',NULL);