This error indicates an expected task was not found during an operating system call that accesses a special semaphore type called a Resource.

These special semaphores have been in use for a long time so it is surprising that there would be an issue.

The customer experiencing the issue began having it after upgrading to XL2CL v4.73.00 from v4.71.00. The error, when it would occur, happened consistently in the same place in the software when accessing a resource used in MODBUS communication. Software comparison showed a change to the MODBUS communication class documented in SCN 4448. This change was added in V4.72.00. Some of the high level communication functions received an additional data protection Resource, nested around the original lower level communication port protection Resource. This indicates there may be a limitation either with nesting Resources or not consistently nesting them.

In order to eliminate either possibility, the original low level port protection Resource was moved to the high level communication functions to protect both the data and the port, eliminating the use of nested Resources in the MODBUS communication.

This is expected to maintain the fix that was introduce in SCN 4448 while eliminating the potential for the Task Error that appears to have been introduced by the same change. This will buy time to investigate and possibly fix the limitations in the Resource type semaphore.