Did you recently update to Spring Boot 3.5.1 or Tomcat 10.1? If so, this might be related to stricter limits for multipart requests.
You can change the limit (e.g. from 10 to 30) by setting the following property:
server:
tomcat:
max-part-count: 30
OR
server.tomcat.max-part-count=30
See https://github.com/spring-projects/spring-boot/releases/tag/v3.5.1