When trying to enable some Language Packs, we faced the following error:
Initially, I thought it was related to a rollup install as I found quite a few peoples with the same symptoms after applying a rollup and the customer did have applied a new rollup.
So, we spent a bit of time uninstalling everything back to UR6, reinstall the MUI and enable them from that state. But, no luck the problem was still present.
At that point, I had a closer look at the error message/stack and the activation fails when trying to upload the new MUI reports and specifically points to the categorycode attribute of the report category entity.
Out of the Box, the categorycode as 4 values pairs (1-Sales,2-Services,3-Marketing,4-Administrative).
I had a look in the entity fields definition and only 2 of the OOB values pair were present in the Option Set
As this attribute is editable, we just had to recreate the missing values pairs and the activation of the MUI Language Packs worked like a charm.
Ideally, those OOB values should not be removable but, until this is fixed, customers should be informed never to remove those default values as it will fails all subsequent MUI Language packs activation.