Failed to solve constraints.

[andy20170608]

Failed to solve constraints when i run the following scripts:
sudo ./probe_dpi.py -G -p 20 -P -N 50 -S 1 ../data/concrete_examples.20190611.sample

The typical log like this:
15:05:40 [INFO] Solving constraints...
15:05:40 [DEBUG] params: {'tcp_options1': 0, 'tcp_seq_num1': 0, 'tcp_doff_reserved_flags1': 144, 'tcp_flags1': 194}
15:05:40 [DEBUG] ####### Cannot solve constraint! #######
15:05:40 [DEBUG] Seems we are overconstraining, we need to relax the constraint...
15:05:40 [DEBUG] Removed constraint on tcp_seq_num1.
15:05:40 [INFO] Solving constraints...
15:05:40 [DEBUG] params: {'tcp_options1': 0, 'tcp_doff_reserved_flags1': 144, 'tcp_flags1': 194}
15:05:40 [DEBUG] ####### Cannot solve constraint! #######
15:05:40 [DEBUG] Still not working, relaxing more constriant...
15:05:40 [DEBUG] No more constraints to relax, giving up...

[gkso]

Which test case is it? It could happen occasionally, but not often.