It seems, that you have read Otto’s outdated instruction. It is 2,5 year old and written for an old version of this plugin. You’re using newest one.

Read ‘Instruction’ for this plugin again. It says in one chapter: “Illustrated installation instructions can be found here but you can ignore the instructions to place domain_mapping.php in mu-plugins”.

Solution to your problem should be deleting “domain_mapping.php” from “mu_plugins” folder or remove whole this folder. Newest version of this pluging doesn’t require it.

I just wrote, what I saw, basing on my own experience. I too followed Otto’s example, copied “domain_mapping.php” to “mu_plugins” and got exactly the same error message. Removing “domain_mapping.php” from “mu_plugins” solved the problem.

If you think, that above mentioned error has different source than placing “domain_mapping.php” in “mu_plugins”, then maybe this is some kind of coincidence.