CloudFormation error: "only 1 subschema matches out of 2"

Question

I want to update the stack, get this error message:

Properties validation failed for resource RDSDBinstance with message: #: #: only 1 subschema matches out of 2 #/DBSubnetGroupName: failed validation constraint for keyword [pattern]

I'm very confused with #: # part of the message, google shows there's usually a kind of path pointing to the place with error. No errors in CloudTrail. More confusing, I have 5 environments already which already got this update successfully.

The new code:

  subnetDatabase0:   # similar for the second subnet, just 
                     # another AZ and third octet of the CidrBlock
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: !Sub "${Cidr}.14.0/24"
      AvailabilityZone: !Select [ "0", !GetAZs "" ]

  RDSSubnetGroup:
    Type: AWS::RDS::DBSubnetGroup
    Properties:
      DBSubnetGroupName: !Sub "${ProjName}-db-subnetgroup"
      DBSubnetGroupDescription: !Sub "${ProjName} DB subnet group"
      SubnetIds:
        - !Ref subnetDatabase0
        - !Ref subnetDatabase1

  sgDBInternalAccess:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupName: !Sub "${ProjName}-db-internal"
      GroupDescription: Enable internal access to DB
      VpcId: !Ref VPC
      SecurityGroupIngress:
      - IpProtocol: tcp
        FromPort: 5432
        ToPort: 5432
        CidrIp: !Sub "${Cidr}.4.0/24"

  RDSDBinstance:
    Type: AWS::RDS::DBInstance
    DependsOn:
      - DbUser
      - DbPwd
    Properties:
      DBInstanceIdentifier: !Ref SanitizedDomain
      DBSubnetGroupName: !Ref RDSSubnetGroup
      DBName: classerium_backend
      DBInstanceClass: !Ref PgInstType
      AllocatedStorage: "20"
      Engine: postgres
      EngineVersion: !Ref PgVersion
      MasterUsername: !GetAtt DbUser.response
      MasterUserPassword: !GetAtt DbPwd.passw
      StorageType: gp2
      BackupRetentionPeriod: 30
      CopyTagsToSnapshot: True
      DeleteAutomatedBackups: False
      VPCSecurityGroups:
        - !Ref sgDBInternalAccess

The only difference with the old code is this part was added

  StorageType: gp2
  BackupRetentionPeriod: 30
  CopyTagsToSnapshot: True
  DeleteAutomatedBackups: False

What am I doing wrong?