[horde] New install of webmail edition 5.1.17 gives blank squares without words/links on top of admin config pages

[info at interpost.biz]

  Hello,
New install of webmail edition 5.1.17 on debian stretch with php 7.2  
and 7.3 and 7.1 gives blank squares (instead of the clickable tabs we  
should see) without words/links on top of admin config pages. The  
exact same number of squares as the rectangles containing words  
(configuration settings tabs) on a working instance. We installed  
horde as usual from stretch repos.

Are we missing a piece of software? What?  
Are those tabs images? Is this a display image problem? 
Is there a configuration setting that needs to be adjusted?

We have a working instance of the same version of horde webmail  
edition on a same server setup and we can't find the diffference  
between them. This appears to be the only problem. Any help  
appreciated. We are modifying the /etc/horde/*/conf.php files manually  
to get it working, but we'd like to be able to use the GUI to make  
small changes, and we can't even see the tabs (except as blank small  
squares) and only the first page of each piece of horde (i.e. horde,  
imp, tream, etc ... displays) in the administration --> configuration   
section.

Thanks for your suggestions of fix.
 

  Ed