I've been importing some CHM projects from RoboHelp HTML X5 into Adobe RoboHelp HTML 8 and it has mostly been going smoothly, but I had one project that would repeatbly hang while it is updating HTML files.
I used a divide-and-conquer approach to track it down to a specific topic. This topic contained the equivalent of the following malformed HTML:
(The actual file had more rows and columns, but still the colspan in the first cell of one row was more than the total number of cells in any row above it.)
This appears to have been written by RoboHelp X5 when it was converted from Word source. That source contained merged cells and misaligned columns (but not that misaligned - just two different sets of column widths, and the cell in question spanned two columns and part of a 3rd from all the rows above it).
Stripping out this colspan attribute (which in the actual file left the long row with fewer cells than the rows above it, rather than more) allowed ARH8 to convert the project without issue.