The workspace format changed with versions of R >= 3.5.0. You might serialize with the newer version 3 format while Exasol requires the previous format 2.
Inside Exasol we are currently running a version < 3.5.0 of R which causes unserialize to fail with above error.
The serialize function in R allows you to specify the workspace version, see here. Hence, if you specify version 2 accordingly then a model created in such a way will be unserialisable in Exasol without error. Here is an example: