Mysteriously the problem reappeared suddenly.
Adding META-INF/ra.xml with this content
<?xml version="1.0" encoding="UTF-8"?>
<connector xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/connector_1_6.xsd"
version="1.6">
<display-name>Filesystem</display-name>
<vendor-name>20-20 Technologies GmbH</vendor-name>
<eis-type>Filesystem</eis-type>
<resourceadapter-version>3.5</resourceadapter-version>
<resourceadapter>
<resourceadapter-class>biz.mbisoftware.fn.jca.filesystem.FilesystemResourceAdapter</resourceadapter-class>
</resourceadapter>
</connector>
the problem can be worked around.