I have removed now the portal in an testdomain with:
univention-app register univention-portal --undo-it
apt-cache madison univention-portal
univention-portal | 3.0.3-1A~4.4.0.202012151822 | https://darkdevil.osit.cc/univention-repository/4.4/maintained/component 4.4-7-errata/all/ Packages
univention-portal | 3.0.2-14A~4.4.0.202011181432 | https://darkdevil.osit.cc/univention-repository/4.4/maintained 4.4-7/all/ Packages
univention-portal | 3.0.2-8A~4.4.0.202008041213 | https://darkdevil.osit.cc/univention-repository/4.4/maintained 4.4-6/all/ Packages
univention-portal | 3.0.2-6A~4.4.0.202005261326 | https://darkdevil.osit.cc/univention-repository/4.4/maintained 4.4-5/all/ Packages
univention-portal | 3.0.2-5A~4.4.0.202005112014 | https://darkdevil.osit.cc/univention-repository/4.4/maintained 4.4-5/all/ Packages
univention-portal | 3.0.2-4A~4.4.0.202004141149 | https://darkdevil.osit.cc/univention-repository/4.4/maintained 4.4-5/all/ Packages
univention-portal | 3.0.2-2A~4.4.0.201909222024 | https://darkdevil.osit.cc/univention-repository/4.4/maintained 4.4-2/all/ Packages
univention-portal | 3.0.1-32A~4.4.0.201907092022 | https://darkdevil.osit.cc/univention-repository/4.4/maintained 4.4-1/all/ Packages
univention-portal | 3.0.1-20A~4.4.0.201903051158 | https://darkdevil.osit.cc/univention-repository/4.4/maintained 4.4-0/all/ Packages
univention-portal | 2.0.2-13A~4.3.0.201810301200 | https://darkdevil.osit.cc/univention-repository/4.3/maintained 4.3-3/all/ Packages
univention-portal | 2.0.1-2A~4.3.0.201804261419 | https://darkdevil.osit.cc/univention-repository/4.3/maintained 4.3-1/all/ Packages
univention-portal | 2.0.1-1A~4.3.0.201803101157 | https://darkdevil.osit.cc/univention-repository/4.3/maintained 4.3-0/all/ Packages
univention-portal | 1.0.12-13A~4.2.0.201711101401 | https://darkdevil.osit.cc/univention-repository/4.2/maintained 4.2-3/all/ Packages
univention-portal | 1.0.12-10A~4.2.0.201708111600 | https://darkdevil.osit.cc/univention-repository/4.2/maintained 4.2-2/all/ Packages
univention-portal | 1.0.12-2A~4.2.0.201705261335 | https://darkdevil.osit.cc/univention-repository/4.2/maintained 4.2-1/all/ Packages
univention-portal | 1.0.12-1A~4.2.0.201704022024 | https://darkdevil.osit.cc/univention-repository/4.2/maintained 4.2-0/all/ Packages
apt-get install univention-portal=3.0.3-1A~4.4.0.202012151822
systemctl restart apache2
systemctl restart univention-portal-server.service
And it worked as intended. Now i have done this in my productive domain. Done, no Errors, but Portal not accessable anmore. It shows me only an small frame on the upper side.
Did I forget something?