It certainly appears that you have done everything correctly. There remains some possibilities however: (i) Ldr.bin is faulty, or (ii) you have a space at the end of the first line of update.txt (i.e. "rrgn,154,2:/154.bin "), or (iii) your microSD is flaky. Here's a zipped Ldr.bin from V8.60 HWID 1948:
Spoiler:
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The MD5 of the extracted Ldr.bin should be 504C567BA05732864EDAB21984185D5D

Try another card if your Ldr.bin and rrgn command are both ok.