Thank you for your interest!

It is not a bug of new versions of FET (5.13.4 or 5.14.0). It is a bug of the input file and a bug of old versions of FET (5.10.3).

The group 773_g1 contains subgroup 2073_p1_g1. This is the first appearance of this group, in year 135_stream.

Then, group 773_g1 contains subgroup 2075_p2_g1. This is the second appearance of this group, in year 121_stream.

This is incorrect. If a students set appears in more places, it must have the same constituents (so, if a group appears in more places, it must have the same subgroups). FET (newer, corrected versions, 5.13.4 and 5.14.0) will only consider the first appearance, and will consider that the next appearances will be identical.

The subgroup 2075_p2_g1 is not read by FET 5.14.0, because it is illegal. It is read by old FET-5.10.3, because FET-5.10.3 contained this minor bug.

The activity in question contains this subgroup, 2075_p2_g1, this is the problem.

So, newer FET (5.13.4 or 5.14.0) contains this bug fix, that is why the file is now unreadable.

Please let me know your opinion and whether you managed to solve this problem (I am interested).