I have uncompressed the archive and tested the file in today's FreeCAD 0.19 from the daily PPA, the object called "OriginalShell", which you appear to think you had converted to a solid, however
1)it is still a shell and
2) it does not pass the Part WB check Geometry test, it contains errors.
I have closed the ticket for now so it can be triaged in the forums first since it appears at this stage to be more of a help request rather than a bug. We can open this ticket again if a bug is found.
Please triage suspected bugs in the Help or Open forums first, since over the years we have found that a significant number of suspected bugs are some sort of user miss-understanding etc.
If you post the FCstd file in which the object called "OriginalShell" was created, then possibly we could find out why it is not a valid shell.
OS: Ubuntu 18.04.5 LTS (KDE/plasma)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Build type: Release
Python version: 3.6.9
Qt version: 5.9.5
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/Australia (en_AU)