The relevant code causing the problem seems to be this from my view:
<input type="checkbox" id=<%= "#{a[0]}#{ndx}" -%> class="toggle" />
<label for=<%= "#{a[0]}#{ndx}" -%> class="popup-label">
This throws the following error in the Heroku logs:
SyntaxError - /app/views/index.erb:25: syntax error, unexpected ')'
2017-03-17T06:30:26.877056+00:00 app[web.1]: ...buf.concat(( "#{a[0]}#{ndx}" -).to_s); @_out_buf.concat " cl...
2017-03-17T06:30:26.877061+00:00 app[web.1]: /app/views/index.erb:26: syntax error, unexpected ')'
2017-03-17T06:30:26.877057+00:00 app[web.1]: ... ^
2017-03-17T06:30:26.877061+00:00 app[web.1]: ...buf.concat(( "#{a[0]}#{ndx}" -).to_s); @_out_buf.concat " cl...
I initially thought it might be the difference between using WEBrick and Puma, but I swapped them and found no difference. Suggestions?