Well, it turned out that I had made changes in the other 2 files unintentionally (dictionary.h or speller.c).
I incorporated most of the suggestions UpAndAdam posted, but ultimately the solution was simpler and DinoCoderSaurus really helped me troubleshoot the issue. I downloaded speller.zip again and pasted the code in my dictionary.c file and the issue is resolved (check50 now says my code compiles and passes all the tests).
I have to admit I feel quite stupid, as I have been changing things in my code for two days now and I hadn't tried the easiest solution, which was just checking if the other two files of the program were intact. It seems that I had changed something in there while reading the code without realizing.
Big thanks to @UpAndAdam and @DinoCoderSaurus that took the time to help me solve the issue!