Mimosa changed their hardware architecture in version 5. Firmware 2.8.1 is legacy software for older hardware revisions. Attempting to "downgrade" a newer C5c to version 2.8.1 will likely "brick" the device , rendering it unusable and requiring an RMA.
Even with a stable release like 2.8.1, hiccups happen. Here are two common issues and their fixes: mimosa c5c firmware 281 download work