ORA-31693: Table data object "SH"."SALES_REG1_Q1" failed to load/unload and is being skipped due to error: ORA-29913: error in executing ODCIEXTTABLEOPEN callout
After some searching on the internet, I found evidence of bug 8393456, and it is indeed confirmed on the Oracle Support website.
The problem seems to be related to cases where you use both PARALLEL (>1) and REMAP_SCHEMA at the same time.
Oracle versions affected are 11.2.0.1, 11.1.0.7 and 10.2.0.4.
I implemented the workaround by setting PARALLEL=1 and I can confirm that it solves the problem.