Upgrading from 14.1.0.3 to 15.0.1 Virtual

May be too late to answer  , but I came across a very similar if not the same problem upgrading from 13.x to 15.0.1, hopefully this helps someone down the line.

Traced it to an original get_dossier error not long after bootup that looks like this:

err get_dossier[8484]: 01170005:3: /config/ssl/ssl.csr/f5_api_com.csr stat fails: No such file or directory.

This error precedes the symptomatic looping errors of:

• error mcpd Configuration Error: MCPProcessor::check_inilitization: The input string contains invalid characters
• Waiting for mcpd without a response

which misguides the search.

At the tail end of the solution for the original get_dossier error, it suggests verifying the license.

https://support.f5.com/csp/article/K70420769

Usually, after an upgrade, if a unit requires re-licensing, it would pop you back to the re-licensing screen on the gui after the initial boot in the new version, so I don't think it dawned upon me to do a license check.

sure enough, checking the license shows it to be inoperational

# tmsh show /sys license

Can't load license, may not be operational

apparently, this invalid license problem messes things so bad in this version, that even the gui can't load

so, now you that can't get your warm fuzzy gui to relicense, you have to follow :

https://support.f5.com/csp/article/K2595

to manually relicense.

Followed the steps, and waited for the system to sort itself out and it actually came back online.

 good luck!