As discovered in a later post, FESAPI v1.* versions has a well known bug about WellboreMarker support in JAVA and C#.
It is fixed in v2.0.0.0 : See JAVA or C# WellboreMarker sometimes crash the library · Issue #223 · F2I-Consulting/fesapi · GitHub
Listed in bug fix in v2.0.0.0 : Release v2.0.0.0 · F2I-Consulting/fesapi · GitHub
A workaround for v1.* has also been discussed in the later post : JVM gets crashed after some minutes(3 to 5 mins) of creating WellboreMarker Instance - #4 by philippeVerney