Actually Vijeo Citect reads digital addresses in blocks of 16 bits. So if there is a digital tag at C1000, Citect will read from C993 to C1008 in a single request. If the address C993 does not exist then 'address out of range' error comes. Because of this optimization of digital read requests make sure that there are valid addresses in the PLC. In some PLCs the valid ranges of all memory addresses can be set and make sure that the setup in the PLC is what Citect is going to read. If the range cannot be adjusted then do not use any digital tags within the first and last 16 digitals.
Vijeo Citect also tries to block reads together and may try to read across invalid addresses. So if C1000 and C1100 are configured, Citect will try to read from C993 to C1105. If any address is invalid between these tags then also 'address out of range' error comes. This is quite unlikely as almost all PLC contain continuous memory with no holes.