I tried to replicate this bug and it seems I have found a possible cause. Whenever the lyrics field contains any 16-bit character (e.g. U+266D Flat “♭”, U+2010 Hyphen “‐”, a lot of Latin letters with diacritical marks), possibly due to non-QWERTY keyboards, saving fails and Hookpad reports the “song data is empty” error. Even when saved as .hkt files, loading these will crash Hookpad completely. Example (U+0056 U+2076 U+2084)
8-bit characters outside the ASCII range cannot be saved at all and Hookpad just gets stuck, but they work properly as .hkt files.
Moreover, the check does not notify the user at all if another Theorytab with the same name exists, after which the title bar becomes glitched. The existing Theorytab won’t be overwritten; the two buttons to the left in the screenshot don’t show anything at all when clicked.