I think Jean Louis is right: it _is_ an org issue. The problem is that the default value of org-latex-default-figure-position is "htb", which is a very bad idea. The string always needs to contain a "p" in it somewhere, otherwise latex will never be able to find a solution for figure placement in cases with many floats and little or no text.
Fuller explanation: The position specifiers "h" (here), "t" (top), and "b" (bottom) are all for placing floats on a page that has some "normal" text on. For all 3 cases, there is a minimum fraction of the page that must be occupied by text (controlled by the parameter \textfraction, which by default is 0.2). To get pages that are entirely occupied by floats, you need to use the specifier "p" (page). So, by just using "htb" you are giving latex a set of requirements that are impossible to satisfy if there are many floats and no text. Hence the problem with "Too many unprocessed floats". I would recommend that the default be "htbp" instead.