Error Message "too large" on upload of image-files

If you want to upload a new file (e.g., a PNG image) in the root folder or in any folder within the project, a message appears saying that the file is too large. However, the file itself is small (150KB). The filename is compliant (all lowercase letters, no special characters, no spaces). Even after closing the browser and trying to upload again, the error appears.
Is there a solution to this? Thank you very much.