When a stack is initialized for each task, I noticed that the R14 register is initialised with value '0xaaaaaaaa'. Has this any meaning or is it dummy value? The R14 is the link register, thats why I asked.

The 0xaaaaaaaa is a dummy value as you suggest. All the registers are filled with dummy values to assist debugging.

As a task function should never exist the value loaded into the link register is never used and can take any value.

Regards.