routeEnhancers as a standalone URL

[firmenpunkt]

I got the routeEnhancers with filters working (TYPO3 11 ke_search 5.0.2), but the generated URL does not work if I reload it, or copy-paste it to another browser etc.
I implemented the "full example" in https://docs.typo3.org/p/tpwd/ke_search/5.0/en-us/Configuration/RoutingSpeakingUrls.html
The search works, The URL looks like this: www.example.com/productsearch/score/desc/0/1/////caregory1

The error message if I reload the page is:

(1/1) #1476107295 TYPO3\CMS\Core\Error\Exception
PHP Warning: Cannot assign an empty string to a string offset in /path../typo3conf/ext/ke_search/Classes/Lib/SearchHelper.php line 313

If I put dashes to the filters like

      filter_1: '-'
      filter_2: '-'
      filter_3: '-'
      filter_4: '-'

the error message changes to:

Parameter "tx_kesearch_pi1__filter_4" for route "enhancer_tx_kesearch_pi100000000174b4617000000007c19c61d" must match "[0-9a-zA-Z-]*" ("- -- -" given) to generate a corresponding URL.

How can I make these URLs work?