Problems with WordPress search

Hello community,

I have the following problem with my WordPress-based website which you can find here: https://arneteubel.com/kunden/jdib

The sitewide search doesn’t find all search terms even though the words/terms are right there.

For example: if you search “polen” or “policy paper” you get no results even though the word appears in one or more articles.

I already installed “Relevanssi” (a search plugin) to see if that improves the situation but it runs into the exact same problems with certain search terms. I talked to the developer and it doesn’t seem related to the plugin as I have the same problem without.

Here is the code for my “search.php”: https://pastebin.com/BnxthUPf

Does anybody have an idea how to handle this and what might go wrong. I could imagine that there is some error in my theme or files so that only parts of my website get searched or indexed. Any help is much appreciated as I have my customer waiting for a solution on this.

Thanks a lot and best regards,
Arne