HTTP 500 INTERNAL SERVER ERROR when I try to start Brewing

I've a freshly installed CBPi. (Commiter Johannes, Commit 4d574b12784bcec3c07a07ac319abfb55a7cb92)

I was able to configure my sensors, actors and kelltes. Also I was able to configure a mash-plan in the "Brewing-tab" without any problems. Finally I'm able to play (manually) with the actors on the Bewing-dashboard (start heating and so on). Anything looks pretty fine so far. Only one problem: Nothing happens, when I click on the "play-button", to start the brewing-plan. My browser (Chrome) receives a http 500 status fom the CBPi. No errors where logged in the app.log.

Request URL: http://10.10.10.220/api/step/start
Request Method: POST
Status Code: 500 INTERNAL SERVER ERROR
Remote Address: 10.10.10.220:80
Referrer Policy: no-referrer-when-downgrade
Connection: keep-alive
Content-Length: 291
Content-Type: text/html
Date: Wed, 28 Aug 2019 14:31:07 GMT
Accept: application/json, text/plain, */*
Accept-Encoding: gzip, deflate
Accept-Language: de,en;q=0.9
Cache-Control: no-cache
Connection: keep-alive
Content-Length: 0
DNT: 1
Host: 10.10.10.220
Origin: http://10.10.10.220
Pragma: no-cache
Referer: http://10.10.10.220/ui/
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/76.0.3809.132 Safari/537.36

Manuel83 / craftbeerpi3

HTTP 500 INTERNAL SERVER ERROR when I try to start Brewing #232