[horde] Blank configuration tabs

[Jaap Winius]

Hi folks,

Recently I installed Debian buster on a new VM and then Horde  
Groupware 5.2.22 on it, including IMP 6.2.22. I logged in as admin to  
configure it and everything seemed fine at first, but then I noticed  
that in the configuration section, for application items where the  
configuration options are split into tabs, that the tabs are all  
blank. There seem to be the same number of tabs per application as  
with my Horde 5.2.17 installation on Debian stretch, but now they all  
have the same minimum width, no text and clicking on them does nothing.

Could this be due to a bug, or is it possible that I'm missing a package?

Thanks,

Jaap