[quote]I changed the checksum, changed the header length etc[/quote]
As last you've quotted, you still have incorrect checksum. Just repeat a checksum fix step.
[quote]srec_cat: test.out: 2874: contradictory 0x00018000 value (previous = 0xFF, this one = 0xC5)[/quote]
This messages says that you have different bytes in the different sources at the same address 0x18000. You may use "-MULTiple" option to make this error to be a warning. Probably, first input fills a region with 0xFF values and second input tries to overwrite them with meaningfull data. Pay attention that "-multiple" option may lead to unpredictable result.