I literally have the same problem as this person: 

Same error message and everything. I clear out my config_global.php so that I can just have a fresh install on my dev computer. But invision is assuming a default username of blank according to the error message. I am thinking this is a bug but I am unsure. There may be a workaround which is to copy over my database on my live site, but it would be a pain for me to do so. It's much cleaner and easier to do a fresh install for testing purposes.

Any ideas?

Ok just figured it out this morning (last night was difficult because there were all kinds of problems with my WAMP setup as it was 2 years old!). PHP had to be upgraded in order to get my 4.x test install working properly, but I was not noticing that each time I went to click "INSTALL", it was giving me localhost//admin/install with 2 double slashes.  But that is what was giving me the headache, and I am sure this person's issue that I linked to had the same problem as me. Removing the double slash was the quick and easy fix.

Edited April 15, 20231 yr by chopin