Solution : https://service.sap.com/sap/support/notes/532976 (SAP Service marketplace login required)
Summary : 
      When upgrading SAP systems with source releases at or below 4.5B, stops might occur during TOOLIMP*, XCNV_DUMP, or DUMP_XCNV phases due to misleading error entries in R3trans logs, which are actually trace entries from database interface changes. To resolve, repeat TOOLIMP* phase or update tp patches from SAP Service Marketplace for designated releases (4.5B, 4.0B, 3.1*). For issues in XCNV_DUMP or DUMP_XCNV with source releases <=3.1I, utilize an updated R3trans post-June 30, 2003. Always replace tp or R3trans executables in specified directories before starting upgrades. Use corrected tp versions for regular transports to avoid similar log issues.
Key words : 
directory /usr/sap/put/preexe, /usr/sap/put/preexe directory, sap service marketplace software center, sap service marketplace, r3trans-import-log, r3trans-export-log, r3trans support package, confusing log entries, -mentioned tp versions, regular tp patch
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