LanguageTool cannot start, error: "dictionary file is supposed to be named *.dict"

We today had a very strange problem with the tool: It crashed and an attempt to restart lead to the error:

Exception in thread "main" java.lang.IllegalArgumentException: dictionary file is supposed to be named *.dict: '../hunspell-dictionaries-for-languagetool/af_ZA/af_ZA.dic'
	at org.languagetool.server.HTTPServerConfig.addDynamicLanguages(
	at org.languagetool.server.HTTPServerConfig.parseConfigFile(
	at org.languagetool.server.HTTPServerConfig.<init>(
	at org.languagetool.server.HTTPServer.main(

A second attempt 15 min later and the service started without problems (?)
We are using Version 4.9 and add hunspell dictionaries in our startup call