I do not have a definitive answer to your problem, however when I go to " http://www.dhl.de/de/privatkunden.html
" using my current version of SeaMonkey 22.214.171.124 I do not have a problem with the page; it shows up just like it does it does in Firefox and in other browsers---and yes I am using uBlock Origin too, and also Privacy Badger.
So try this experiment: it seems that you already know how to get to "about:config"; so once you get there scroll down or search until you get to the previously suggested "general.useragent.override" area. If you have anything listed there for "general.useragent.override" then right click on it, click "modify", delete your present string value, and replace it with "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Firefox/60.0", then click okay. [You might want to save the previous string value in case you wish to revert back to it.]
In addition if you have any other items for "general.useragent.override" with special pages designated at the end, such as "general.useragent.override.google.com", right click on those entries and then click reset. This will delete the string value and the next time the you close and re-open SeaMonkey that listing will no longer be there. [Again, you might want to save the previous string value(s) in case you wish to revert back to it/them.]
The result of these operations will leave you with only one entry for "general.useragent.override", the one that I suggested above that works on my computer for the web page that you have referred us to. Previously I was doing overrides for specific web pages but I have found that this particular string value works for all of the pages where I have had problems before, so I have managed to get by without any page-specific overrides so far.
As indicated save all of your previous values just in case you wish to revert back to them.