I have exactly the same situation:
One form works, another not.
After a lot of testing i found the difference in using Html-Content-Type for mail body or not. Disabling Html-Content-Type stopped spinning arrows after sending mail.
I think there must be a bug in the Contact-Form code.
One thing i found out:
The following HTML code worked with Html-Content-Type-Body:
....
<td colspan="3">
<div style="margin-left: 2em;">IBAN: ....
this worked not:
....
<td colspan="3">
<div style="margin-left: 2em;">
IBAN: ....
The only difference ist the linefeed after div-tag.
Please fix the bug.
Thanks a lot !