I think the limit is 9 characters on the Z-machine (but can be less if there are special characters which use an encoding that takes up more space than usual), and it’s configurable on Glulx with the setting “Use DICT_WORD_SIZE of 15.” (or another number, of course).
See chapter 2.2 Varying What Is Read in the Recipe Book:
by default Inform truncates words to nine letters before attempting to identify them. This is no problem in most circumstances and is likely to go unnoticed – until we have two very long words whose names are nearly identical, such as “north-northwest exit” and “north-northeast exit”. (To make matters worse, a punctuation mark such as a hyphen counts as two letters on its own.)
When we are compiling for Glulx, the limit is easily changed with a single line, setting the constant called DICT_WORD_SIZE.
Here are other threads on input length issues, which mention limits of 20 words and 260 characters in total:
Also, here’s a (very old) thread where a similar error message about “BlkValueWrite: writing to index out of range” crops up, where it probably has to do with needing to choose a row before modifying a table entry. But that might be totally unrelated to your question, since BlkValueWrite is used in all sorts of places (I think).
I don’t know if any of this sheds light on the issue you are facing, unfortunately.
If possible, it would be great if you could isolate the problem into a reproducible minimal example. (But I know that’s often really hard to do.)