Error on viewing some emails

Hello my friends :)

I've a really strange error.
I can view the most of my emails in Horde IMP with no errors and no  
problems, but some other mails i get the error "Communication Error  
with the server."

There is no pattern of what mails returning the error...
i looked in the logs.

SYSLOG with HORDE:

domain.com:443 MY-ISP-IP - - [01/Nov/2016:16:53:16 +0100] "POST  
/horde/services/ajax.php/imp/showMessage HTTP/1.1" 500 577  
"https://domain.com/horde/imp/dynamic.php?page=mailbox" "Mozilla/5.0  
(Windows NT 10.0; WOW64; rv:49.0) Gecko/20100101 Firefox/49.0"

It seems to be a Error 500 on the "error mails".

on the apache errorlog, i get this:

[Tue Nov 01 16:57:37.875274 2016] [:error] [pid 7591] [client  
MY-ISP-IP:50189] PHP Fatal error:  Cannot use 'String' as class name  
as it is reserved in  
/usr/share/php/Horde/Css/Parser/vendor/sabberworm/php-css-parser/lib/Sabberworm/CSS/Value/String.php on line 5, referer:  
https://first-s0ul.de/horde/imp/dynamic.php?page=mailbox

On mails that are working, i get this in the syslog:

domain.com:443 MY-ISP-IP - - [01/Nov/2016:16:54:50 +0100] "POST  
/horde/services/ajax.php/imp/showMessage HTTP/1.1" 200 9163  
"https://domain.com/horde/imp/dynamic.php?page=mailbox" "Mozilla/5.0  
(Windows NT 10.0; WOW64; rv:49.0) Gecko/20100101 Firefox/49.0"

Has anyone a idea?

my system:
ubuntu 16.04 with PHP7.0 and apache2 2.4.18
Horde Groupware Webmail Edition 5.2.12
IMP 6.2.12

thanks!
Greetings from Germany!
  MIT FREUNDLICHEN GRüßEN,
BEST REGARDS,

STEFAN FRöCHTENICHT
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