Ok, so you did not change anything extra-ordinary from what you described. Many of us use permalinks options active, so that part is fine.
Plugin should be working, regardless of that setting, as long as everything else on your website runs fine.
However, I am really puzzled about theme having problems with permalinks, what kind of problems you had? Can you explain? Themes really have nothing with that, or they should not, normally. Was the Bender theme working fine? What server do you run Osclass on (LiteSpeed, Apache... reverse-proxy etc.)? All that usually matters in this kind of issues.
On another hand, if plugin uses hard-coded links like that, that usually means bad design, because plugins must take into account different possibilities (with or without permalinks). Maybe, if you contact developer again and explain the issue better, he may understand and fix it in that case.
Of course, this is all speculative and preliminary talk, as I don't know plugin's code.
Regards