This thread is resolved. Here is a description of the problem and solution.
Problem: I am trying to use multi-currencies with Woocommerce multilingual. Since about 24 hours the connection to fixer.io has a problem by some reasons, as all the rest works the reason is supposed to a change in fixer.io. Now fixer.io causes PHP-errors that crash processes which use the multicurrency feature. The only currency that works is USD, even if the base_currency is other (in my case CHF)
Solution: It seems that this issue is already handled in the beta version of Woocommerce Multilingual
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