Steps to reproduce the issue

I just switched Joomla's server PHP version from PHP 5.6.x to 7.1.5.

Expected result

Website including links works as before after clearing cache.

Actual result

When clicking on weblinks, it is stuck at the step, e.g., HTTX//URL/xxx?task=weblink.go&id=35 and doesn't actually go to the link destination (just blank page).
Also in den Admin Backend, I cannot edit links anymore (just goes to blank page as well).

System information (as much as possible)

Joomla Sys Info:

PHP Built On Linux info 3.0 #1337 SMP Tue Jan 01 00:00:00 CEST 2000 all GNU/Linux
Database Version 5.5.55-0+deb7u1-log
Database Collation latin1_german2_ci
Database Connection Collation utf8mb4_general_ci
PHP Version 7.1.5
Web Server Apache
WebServer to PHP Interface cgi-fcgi
Joomla! Version Joomla! 3.7.2 Stable [ Amani ] 22-May-2017 09:46 GMT
Joomla! Platform Version Joomla Platform 13.1.0 Stable [ Curiosity ] 24-Apr-2013 00:00 GMT
User Agent Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:53.0) Gecko/20100101 Firefox/53.0

Joomla PHP Info:
Relevant PHP Settings
Setting Value

Safe Mode Off
Open basedir None
Display Errors On
Short Open Tags On
File Uploads On
Magic Quotes Off
Register Globals Off
Output Buffering Off
Session Save Path /tmp
Session Auto Start 0
XML Enabled Yes
Zlib Enabled Yes
Native ZIP Enabled Yes
Disabled Functions None
Multibyte String (mbstring) Enabled Yes
Iconv Available Yes
Mcrypt Enabled Yes
Maximum Input Variables 5000

Additional comments

I'm using 1und1.de hosting. Website is working fine under PHP 5.6