C - Unlink/Remove produces error for filenames with spaces

Question

I am trying to make a function in C to erase all the contents of a temp folder and to erase the folder.
Whilst I already have successfully created the code to cycle through the files and to erase the folder (it is pretty much straight forward) I am having trouble erasing the files using unlink.
Here is the code that I am using:

int delete_folder(char *foldername) {
 DIR *dp;
 struct dirent *ep;
 dp=opendir(foldername);
 if (dp!=NULL) {
      readdir(dp); readdir(dp);
      while (ep=readdir(dp)) {
           char* cell = concatenate(concatenate(foldername, "\\"), "Bayesian Estimation.xlsx");//ep->d_name);
           printf("%s\n", cell);
           remove(cell);
           printf("%s\n", strerror(errno));

      }
      closedir(dp);
 }
 if (!rmdir(foldername)) {return(0);} else {return(-1);}
}

The code that I wrote is fully functional for all files but those which include spaces in the filename. After some testing, I can guarantee that the unlink functions eliminates all files in the folder (even those with special characters in the filename) but fails if the filename includes a space (however, for this same file, if I remove the space(s), this function works again).

Has anyone else encountered this problem? And, more importantly, can it be solved/circunvented?
(The problem remains even if I introduce the space escape sequences directly)
The error presented by unlink is "No such file or directory" (ENOENT). Mind you that the file is indeed at the referred location (as can be verified by the code outputing the correct filename in the variable cell) and this error also occurs if I use the function remove instead of unlink.

PS: The function concatenate is a function of my own making which outputs the concatenation of the two input strings.

Edit: The code was written in Codeblocks, in Windows.
Here's the code for the concatenate function:

char* concatenate(char *str1, char *str2) {
 int a1 = strlen(str1), a2 = strlen(str2); char* str3[a1+a2+1];
 snprintf(str3, a1+a2+2, "%s%s", str1, str2);
 return(str3);
}

Whilst you are right in saying that it is a possible (and easy) memory leak, the functions' inputs and outputs are code generated and only for personal use and therefore there is no great reason to worry about it (no real need for foolproofing the code.)

Answer 1

Thank you very much for all your input, after reviewing your comments and making a few experiments myself, I figured out that remove/unlink was not working because the filename was only temporarily saved at variable cell (it was there long enough for it to be printed correctly to console, hence my confusion). After appropriately storing my filename before usage, my problem has been completely solved.
Here's the code (I have already checked it with filenames as complex as I could make them):

int delete_folder(char* foldername) {
 DIR *dp;
 struct dirent *ep;
 dp=opendir(foldername);
 if (dp!=NULL) {
      readdir(dp); readdir(dp);
      while (ep=readdir(dp)) {
           char cell[strlen(foldername)+1+strlen(ep->d_name)+1];
           strcpy(cell, concatenate(concatenate(foldername, "\\"), ep->d_name));
           unlink(cell);
           printf("File \"%s\": %s\n", ep->d_name, strerror(errno));
      }
      closedir(dp);
 }
 if (!rmdir(foldername)) {return(0);} else {return(-1);}
}  

I realize it was kind of a noob mistake, resulting from my being a bit out of practice for a while in programming in C, so... Thank you very much for your all your help!