It might be worth trying to recover the file in FileMaker Pro 11 prior to trying to convert your .fp7 file to .fmp12.
Even though the file itself might be "working properly", there could be a corrupt / damaged layout objects on one layout, for example, that is causing the conversion process fits.
And while the symptoms are not the same - you might try referring to this article and make sure the settings in your fp7 file are valid.
If the above does not check out, does FileMaker Pro 14 attempt to recover the file and fail somewhere in the middle? If it does, consult the log file that is generated and see if you can tell at what point the conversion process fails. It might provide insight into something you can modify in your .fp7 file prior to converting the file.
You don't say how you "checked" the file in 11, but you might try a "save a compressed copy" before conversion.
Saving it as a compressed copy worked. It converted properly with no lost data. Thanks!