Dear Ludovic,
Thank you very much for your help! Given that the problem happens for me only while processing huge binary files after around an hour of processing (and the problem in general is not consistent), I am not sure how to come up with a reduced test case that would still reproduce the problem. So, I would appreciate any help with that.
I was also wondering that given we can reproduce the problem under a particular circumstance, if we can approach the problem from the other side - e.g. by making changes on guile side to print information on the hash table for which the assertion is failing, print stats on this hash table over a period of time, make changes to hash table code to try to resolve the problem, etc.