Thanks to anon88919003 and oz-ra i finaly could go from libeelec to the new coreelec.
I own an X96 (s905x) tv box with 2gb ram - wich worked rocksteady with the old libreelec and Kodi Krypton.
After switching to coreelec (for Emulation) the file transfer error occured.
Tested all available bootloaders with no luck - so i did go back to the old libreelec.
Now that rc3 was released - i tried again and failed. Same old transfer error.
But then i found your posts for the le potato (Wich has the same Chip)
and gave it a try. (gxl_p212_2g bootlader)
IT WORKS - no file transfer error until now.
thank you very much