Hi, NEXTTCOD is a site written assembler program to label the CV. This was developed a few years ago to identify the CV among the different environments. NEXTTCOD navigates the control blocks in memory to identify the CPU, the environment and the IDMS CV. Underscore sign (_) means that the navigation in the control was not able to catch the necessary information. In this case the environment TGT is correctly filled in.
I suggest to look in the task dump to identify the bug in NEXTTCOD and to bring some info about RHDCTABL abend.