File status code 90 = One of the many file return codes or cobol file status codes. It is usually returned when the file I-O result was that of a bad open file command. There obviously could be a limitless number of causes for this error but I would start by making sure the file was closed to begin with, or if you are opening and closing files in the cobol program make sure that you closed it before issuing an open command again as that will certainly cause an error.
Although if you are open and closing the same files in one program, then I would suggest this is bad coding as your should break your logic down into several different small programs, rather then having one big program closing and reopening files. Breaking it down into smaller programs will make it far less error prone, and much easier to maintain or test each going forward.
Typically some of the older cobol programs from many years ago using vsam often had dumped alot of logic into one big program and read / write many files out of it, thats completely the wrong way to be coding and makes it a testing nightmare.