Are you getting errors similar to the below in the w3svc logs on your CallManager when the failure occurs?
2001-09-07 19:01:20 172.21.11.155 - 171.69.92.117 80 GET /CCMCIP/getservicesmenu.asp name=SEP00036B8B098A 200 Allegro-Software-WebClient/3.12
2001-09-07 19:01:37 171.69.92.112 - 171.69.92.117 80 POST /LoginService/login.asp - 500 Mozilla/4.0+(compatible;+MSIE+5.0;+Win32)
If so, then you may be hitting this:
CSCdv29329 - EM login service fails
Symptom:
EM login service intermittenly fails
Condition:
EM login service intermittenly fails. The problem seems to be an issue of the virtual directory in IIS failing
for some reason. No files may be returned from this particular virtual
directory by IIS, including standard .htm files, as well as .asp files.
All requests to the virtual directory which request a file receive a
HTTP 500 error (Internal Service Error), without further explanation.
Workaround:
The virtual directory will again function properly once IIS is restarted.
If you are hitting this please open a TAC.