Symptom:
When the IOS PKI Client tries to renew it's existing certificate [i.e. the RENEW timer in "show crypto pki timer" reaches 0], if the first attempt is interrupted due to communication failure [for example: the link between the client and the RA/CA is broken], the subsequent attempts to renew the certificate fail. 

Debugs will show:
[debug crypto pki transaction]
CRYPTO_PKI: Failed to send the request. There is another request in progress.

Because of this, removing the trustpoint also fails with the following error:
Error: There is an auto enrolment transaction in progress.
Please wait until the current auto enrolment to finish before
starting a new enrolment transaction.

no crypto pki enroll _TP-Name_  - Does not help in this case

Conditions:
IOS PKi Client configured to enroll through an RA, with auto-renewal configured:

crypto pki trustpoint TP
  enrollment mode ra
  auto-enroll 80