It seems to be a known problem by Alkacon (OpenCms developer):
Here is a possible cause:
https://github.com/alkacon/opencms-core/issues/73
("pbeltran" is me there.)
If it helps we did the following on OpenCms 8.5 in order to avoid the error, but sacrificing properties files :(
Just edit WEB-INF/config/opencms-importexport.xml and replace with this:
<repository name="standard" class="org.opencms.repository.CmsRepository">
<params>
<param name="wrapper">org.opencms.file.wrapper.CmsResourceExtensionWrapperJsp</param>
<param name="wrapper">org.opencms.file.wrapper.CmsResourceExtensionWrapperXmlContent</param>
<param name="wrapper">org.opencms.file.wrapper.CmsResourceExtensionWrapperXmlPage</param>
<param name="wrapper">org.opencms.file.wrapper.CmsResourceWrapperSystemFolder</param>
<!--param name="wrapper">org.opencms.file.wrapper.CmsResourceWrapperPropertyFile</param-->
</params>
<filter type="include">
<regex>/</regex>
<!--regex>/__properties/.*</regex-->
<regex>/sites/.*</regex>
<regex>/system/</regex>
<!--regex>/system/__properties/.*</regex-->
<regex>/system/galleries/.*</regex>
<regex>/system/modules/.*</regex>
<!--regex>/system/workplace/.*</regex-->
</filter>
</repository>
Afer that restart your server.
We do regular exports from OpenCms of the modules to keep the whole package (files and properties) in order to keep the solution under control version (SVN). You can create a module with a resource pointing to sites/default to do that. But it's critical to select root site before doing any import/export of that module (if you dont you can break your ocms installation).