So with the help of rclone guys I might have found the issue:   I have 12 gb of ram and when I upload + all the services running on unRAID i am using about 8.5 of the ram. When Plex is doing the thumbnails it seems that it consume all remaining ram for the job: it consume some ram for Plex itself + the --buffer-size 256 * number of opened files. Apparently its 4-5 files simultaneously.   I lowered the buffer-size variable to 128mb and I have not seen the issue since 24h.
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