We are having the following issue on the 78xx;
Once a while the alternative log on function does not work properly. If the issue accurs, we can restart the MFP and it works again for a short period of time.
We double checked all our configurations and can not find any obstacels which may cause this issue.
If you have not already resolved this issue, we suggest you search our online knowledgebase for this product: WorkCentre 78xx.
Alternatively, you can visit our Contact Us page for other support options.