The economic crisis in the latter half of 1990s most seriously affected Indonesia, Thailand, Malaysia and South Korea. The cause of the crisis was

The 1997–98 East Asian crisis reflected fundamental weaknesses in the domestic financial systems rather than merely an external recession. Weak prudential supervision, government-directed lending and poor enforcement left banks with deteriorating loan portfolios and high vulnerability to shocks [1]. These fragilities combined with excessive short‑term foreign‑currency borrowing and dangerous maturity/currency mismatches that amplified losses when capital flowed out [2]. Speculative attacks on largely pegged or managed exchange rates then triggered rapid currency collapses and a banking-financial crisis across Thailand, Indonesia, Malaysia and Korea, showing that mismanagement of financial resources and the financial sector was the principal underlying cause.

Sources

1. [1] https://www.imf.org/external/pubs/ft/fandd/1998/06/imfstaff.htm
2. [2] https://www.adb.org/sites/default/files/publication/363326/20-years-asian-financial-crisis.pdf