Error enabling thumbnails on existing and populated directory

[nu111]

I have a dir with several image files.
I set up RichFilemanager to use this dir as the user storage folder .
I set up RichFilemanager to use thumbnails.
When i run RichFilemanager all images previews are broken images and I see this errors on the PHP Log

[19-Feb-2019 13:31:56 GMT] PHP User Notice - /xxx/tinyMCE_upload/_thumbs/Screenshot_20180608_084612.png, /xxx/composer/vendor/servocoder/richfilemanager-php/src/Repository/Local/ItemModel.php, 689
Uri: /adm/js/RichFilemanager-2.7.6/connectors/php/filemanager.php?path=%2FScreenshot_20180608_084612.png&mode=getimage&thumbnail=true&time=1550583115886
Ip: 127.0.0.1
#0 [internal function]: {closure}(1024, '/xxx...', '/xxx...', 689, Array)
#1 /xxx/composer/vendor/servocoder/richfilemanager-php/src/Repository/Local/ItemModel.php(689): trigger_error('/xxx...')
#2 /xxx/composer/vendor/servocoder/richfilemanager-php/src/Api/LocalApi.php(567): RFM\Repository\Local\ItemModel->checkReadPermission()
#3 /xxx/composer/vendor/servocoder/richfilemanager-php/src/Application.php(338): RFM\Api\LocalApi->actionGetImage(true)
#4 /xxx/adm/js/RichFilemanager-2.7.6/connectors/php/filemanager.php(132): RFM\Application->run()
#5 {main}

The problem gets fixed if I upload through RichFilemanager a new image or if I manually create the dir _thumbs.