The function mhttpd_init() scans the custom page and installs handlers etc. for each modbxxx 
element. If you create an modbvalue dynamically in your code, this is probably done after you 
called mhttpd_init(), so the function has no chance to modify the dynamically created element.

To fix that, I separated mhttpd_init() into the old init function which installs the header and sidebar, 
and a function mhttpd_scan() which scans the custom page and processes all modbxxx elements. 
Next, I tapped the error you reported, and added an automatic call to mhttp_scan() in case that 
happens. I tried it on a test page and it worked for me. Please give it a try (commit 090394e8).

Stefan